401. A small nitrogenous waste is passively reabsorbed from tubular fluid into peritubular blood. Its concentration is experimentally made equal on the two sides of the tubular wall, while membrane permeability and cellular ATP remain normal. What is the most likely immediate effect?
ⓐ. Passive reabsorption increases despite loss of the concentration difference
ⓑ. The substance is actively pumped into blood at the same rate
ⓒ. Net passive reabsorption falls because the concentration gradient is lost
ⓓ. Glomerular filtration of all substances stops
Correct Answer: Net passive reabsorption falls because the concentration gradient is lost
Explanation: Passive transport depends on a favourable concentration or electrochemical difference rather than direct expenditure of cellular ATP. When the concentration of the waste is equalised across the tubular wall, molecules may continue moving randomly in both directions, but little net transfer toward blood is expected. Membrane permeability remains intact, so the pathway has not disappeared; the driving difference has. Normal ATP cannot restore passive net movement when no active transporter is involved and the concentration difference has disappeared. Glomerular filtration occurs at the renal corpuscle and need not change when a tubular concentration gradient is altered. Permeability, cellular energy and concentration difference are independent variables whose effects must be interpreted separately in renal transport. The experiment therefore isolates the loss of a passive reabsorptive driving force. It provides no evidence that tubular secretion has changed, and it does not imply that other renal processes have failed.
402. Active recovery of \(\mathrm{Na^+}\), glucose and amino acids in the PCT is strongly inhibited, but tubular water permeability remains normal. The medullary gradient and descending limb are unaffected. Which comparison is most plausible immediately after treatment?
ⓐ. PCT water recovery rises, while descending-limb water loss stops
ⓑ. Water is actively pumped from both regions at an unchanged rate
ⓒ. PCT and descending-limb water movement must both disappear completely
ⓓ. PCT water recovery falls, while descending-limb water loss continues
Correct Answer: PCT water recovery falls, while descending-limb water loss continues
Explanation: Water moves passively and depends on local osmotic conditions. In the PCT, active recovery of sodium and nutrients helps establish conditions favouring accompanying water reabsorption. Strong inhibition of these solute movements can reduce proximal water recovery even though water permeability itself remains present. The descending limb operates in a different environment. Its water loss is driven by the hyperosmotic medullary interstitium, which remains intact. Water can continue leaving that segment. The two passive water movements respond differently because their local osmotic driving forces arise from different segment-specific conditions. Their membranes may both permit water, yet the osmotic conditions generating net flow arise from different segment-specific processes. The preserved medullary gradient explains why descending-limb water movement can continue even while loss of proximal solute recovery weakens osmotic water recovery in the PCT.
403. A toxin selectively destroys the renal cortex while initially sparing most medullary tissue. Which combination of functions is most directly impaired?
ⓐ. Cortical glomerular filtration plus major PCT and DCT transport
ⓑ. Storage of urine in the bladder and its release through the urethra
ⓒ. Pulmonary carbon-dioxide elimination and bile secretion
ⓓ. Only deep-loop counter-current exchange, with filtration remaining normal
Correct Answer: Cortical glomerular filtration plus major PCT and DCT transport
Explanation: Renal corpuscles, PCTs and DCTs are located in the cortex. Extensive cortical destruction therefore damages glomeruli responsible for filtration, PCTs responsible for bulk recovery of water, electrolytes and nutrients, and DCTs responsible for conditional ionic and acid-base adjustment. Although loops of Henle and collecting ducts extend through the medulla, they depend on filtrate supplied by functioning cortical renal corpuscles and earlier tubular segments. Preserving medullary tissue cannot maintain normal urine formation when these upstream cortical components have been destroyed. The bladder and urethra lie outside the kidney, so their storage and release functions are not directly targeted by a cortical toxin. Pulmonary carbon-dioxide elimination and biliary excretion use different organs. The lesion therefore causes a combined failure of cortical filtration and major convoluted-tubule transport rather than an isolated loss of deep-loop counter-current exchange.
404. An excretory organ continues removing nitrogenous waste at its usual rate, but it can no longer adjust water loss or maintain stable concentrations of body-fluid ions. Which interpretation is most accurate?
ⓐ. Both excretion and osmoregulation remain normal
ⓑ. Waste excretion is preserved, but osmoregulation is impaired
ⓒ. Osmoregulation is preserved, but nitrogenous-waste excretion is absent
ⓓ. The organ now performs only egestion
Correct Answer: Waste excretion is preserved, but osmoregulation is impaired
Explanation: Excretion and osmoregulation are related but distinct functions. Removal of nitrogenous waste demonstrates that the excretory component remains active. Failure to regulate water loss and ionic concentrations shows impairment of osmoregulation, whose purpose is to maintain suitable body-fluid volume and solute balance. Many excretory organs perform both roles, so one function may be selectively disturbed while the other remains measurable. Egestion concerns elimination of undigested material from the alimentary canal and does not describe either observed process. The case prevents a common overgeneralisation that any organ capable of waste removal must automatically maintain normal water-ion homeostasis. Functional assessment requires testing both the chemical-waste output and regulation of the internal fluid environment.
405. Examine the following animal records.
| Record | Animal setting | Principal waste and route |
|---|
| P | Aquatic insect | Ammonia lost rapidly into abundant water |
| Q | Marine fish listed as ureotelic | Urea formed after hepatic conversion and removed through kidneys |
| R | Bird | Ammonia stored and eliminated in a large volume of dilute urine |
| S | Land snail | Uric acid eliminated with minimal water |
Which set contains only internally consistent records?
ⓐ. Only records P and R are consistent
ⓑ. Only records Q and R are consistent
ⓒ. Only records P, Q and S are consistent
ⓓ. All records P, Q, R and S are consistent
Correct Answer: Only records P, Q and S are consistent
Explanation: Aquatic insects may be ammonotelic since abundant water supports rapid dilution of toxic ammonia, making P consistent. Many marine fishes are ureotelic; ammonia is converted into urea in the liver, transported in blood and removed through kidneys, so Q is also consistent. Birds are uricotelic rather than animals that store ammonia and release large volumes of dilute urine, making R incorrect. Land snails eliminate uric acid with little water and fit S. The correct set is P, Q and S. The table tests important qualifiers: not every aquatic animal is ammonotelic, and organism-based classification must remain connected to waste toxicity, conversion route, physical form and water economy. The accepted records preserve the link among animal group, nitrogenous product, route and water economy. Each accepted row must remain coherent across habitat, product and water requirement; changing only the animal name cannot preserve the same excretory classification.
406. A terrestrial amphibian normally converts metabolic ammonia into urea before renal elimination. Its hepatic conversion process is blocked, while water availability remains limited and renal structure remains normal. Which problem is most likely?
ⓐ. Toxic ammonia accumulation after loss of hepatic urea formation
ⓑ. Improved water conservation through direct ammonia storage
ⓒ. Immediate conversion of ammonia into uric acid inside the ureter
ⓓ. Complete prevention of all cellular nitrogen metabolism
Correct Answer: Toxic ammonia accumulation after loss of hepatic urea formation
Explanation: Terrestrial amphibians commonly use ureotelism. Conversion of highly toxic ammonia into less toxic urea permits transport in blood and renal elimination with less water than direct ammonotelism would require. Blocking hepatic urea formation leaves metabolic ammonia without its normal detoxifying conversion route. Limited environmental water makes rapid diffusion and dilution of ammonia particularly difficult, increasing the risk of toxic accumulation. Normal kidney structure cannot solve the problem if the principal waste reaching it has not been converted into the form normally handled by this strategy. Ammonia cannot simply be stored safely or converted into uric acid inside a ureter. Cellular nitrogen metabolism may continue, which is precisely why the unprocessed waste becomes dangerous. Continued production of highly toxic ammonia without efficient conversion creates a retention problem that limited water availability cannot solve by rapid dilution.
407. Medullary pyramids are extensively damaged, while most cortical renal corpuscles remain functional. Which outcome best reflects this distribution of injury?
ⓐ. Glomerular filtration stops completely, but urine concentration is unaffected
ⓑ. Only PCT nutrient reabsorption is lost
ⓒ. Urine reaches the bladder normally through cortical columns
ⓓ. Filtration persists, but loop, collecting-duct and calyceal functions decline
Correct Answer: Filtration persists, but loop, collecting-duct and calyceal functions decline
Explanation: Glomerular filtration occurs in renal corpuscles situated in the cortex, so preservation of many cortical corpuscles allows some initial filtrate formation. Medullary pyramids contain descending and ascending tubular portions and collecting ducts travelling toward the calyces. Extensive damage disrupts loop-dependent concentration mechanisms, collecting-duct water handling and delivery of urine from pyramid tips into calyces. Cortical columns are tissue extensions between pyramids and are not alternative urine-conducting channels to the bladder. PCTs are mainly cortical and are not the only structures affected by this lesion. The expected pattern separates preserved upstream filtration from severe impairment of medullary processing and internal drainage. The manipulation tests necessity within the renal cortex and medulla: loss of the component removes its contribution but does not automatically abolish unrelated functions.
408. A tracer is delivered only to the renal medulla and labels loops of Henle, collecting ducts and vasa recta. It does not label renal corpuscles, PCTs or DCTs. Which conclusion is most appropriate?
ⓐ. renal corpuscles lie within medullary collecting ducts and vasa recta
ⓑ. PCTs and DCTs form the medullary pyramids alongside vasa recta
ⓒ. Cortical corpuscles and tubules contrast with medullary loops and ducts
ⓓ. vasa recta are urinary tubules located with cortical corpuscles
Correct Answer: Cortical corpuscles and tubules contrast with medullary loops and ducts
Explanation: Renal corpuscles, PCTs and DCTs are positioned within the cortex. Loops of Henle descend from the cortex into the medulla, collecting ducts pass through medullary pyramids, and vasa recta accompany the long loops of juxtamedullary nephrons. A tracer restricted to the medulla consequently labels these loops, ducts and vessels but not the cortical corpuscles or convoluted tubules. Vasa recta must be kept in the vascular category; they carry blood and participate in counter-current exchange, whereas loops and collecting ducts carry tubular fluid. The regional labelling reconstructs how one nephron spans two kidney zones. Filtration and much convoluted tubular processing occur in the cortex, while loop-related concentration, vasa-recta exchange and collecting-duct transit involve the medulla. This spatial distinction supports the conclusion that cortical corpuscles and tubules contrast with medullary loops, collecting ducts and associated vessels.
409. A nephron has a renal corpuscle in the cortex, a short loop extending only slightly into the medulla and an absent or highly reduced vasa recta. Which statement best describes it?
ⓐ. It is non-functional since every nephron requires a long vasa recta
ⓑ. It is a cortical nephron with limited deep-gradient contribution
ⓒ. It is a juxtamedullary nephron specialised for maximal urine concentration
ⓓ. It is a collecting duct receiving fluid from many nephrons
Correct Answer: It is a cortical nephron with limited deep-gradient contribution
Explanation: Most nephrons are cortical nephrons with relatively short loops of Henle that extend only a small distance into the medulla. Their vasa recta are absent or highly reduced. They still possess functional renal corpuscles and tubules and participate in filtration, reabsorption and secretion. Their limited loop depth and vascular development make them less important than juxtamedullary nephrons in establishing the deep corticomedullary osmotic gradient required for maximal urine concentration. A renal corpuscle in the cortex does not by itself distinguish the types, since renal corpuscles are cortical in both. Loop length and vasa-recta development provide the decisive evidence.
410. A person has increased body-fluid volume, marked atrial stretching and reduced ADH stimulation. At the same time, narrowing of a renal artery lowers GFR in one kidney. Assuming each sensor responds to its own signal, which pattern is possible?
ⓐ. Low ANF, low renin and high ADH
ⓑ. High ANF, high renin and low ADH
ⓒ. High ADH, low ANF and low renin
ⓓ. Low ADH, low ANF and complete absence of renal feedback
Correct Answer: High ANF, high renin and low ADH
Explanation: Increased body-fluid volume suppresses osmoreceptor stimulation and reduces ADH release. Marked atrial stretching independently stimulates ANF, which favours vasodilation and opposition to excessive fluid retention. Local narrowing of a renal artery can reduce glomerular flow, pressure and GFR in the affected kidney, activating its juxtaglomerular apparatus and increasing renin release. These signals can coexist even though some of their downstream effects oppose one another. The combined pattern arises because atrial stretch, systemic osmotic status and local renal perfusion are detected at different sites. Body-fluid volume, atrial filling and local renal perfusion are not identical variables. Each regulatory system responds to the condition it monitors, producing the combined pattern of high ANF, high renin and low ADH.
411. Secretion of \(\mathrm{H^+}\) is selectively blocked in the PCT, DCT and collecting ducts, while water transport, \(\mathrm{K^+}\) handling and \(\mathrm{HCO_3^-}\) reabsorption remain functional. Which change is most directly expected?
ⓐ. increased tubular acidification despite blocked hydrogen-ion secretion
ⓑ. loss of pressure-driven glomerular filtration despite intact corpuscles
ⓒ. conversion of tubular fluid into blood within collecting ducts
ⓓ. reduced ability to maintain the normally slightly acidic urinary pH
Correct Answer: reduced ability to maintain the normally slightly acidic urinary pH
Explanation: The PCT, DCT and collecting ducts can all add hydrogen ions to tubular fluid. This shared secretory activity contributes to acid-base regulation and helps produce urine that is normally slightly acidic, near \(\mathrm{pH}\,6.0\). Selectively blocking hydrogen-ion secretion weakens this acidifying influence even though other transport processes remain intact. Preserved bicarbonate reabsorption, potassium handling and water movement keep the defect focused on tubular acidification rather than complete nephron failure. Glomerular filtration occurs at the renal corpuscle and does not depend directly on hydrogen-ion secretion in downstream segments. The experiment demonstrates that several nephron regions cooperate in acid-base adjustment and that their common transport direction has a measurable effect on final urine chemistry. With less hydrogen ion entering tubular fluid, final urine tends to become less acidic even though other water and electrolyte movements continue.
412. Bladder stretch receptors detect filling normally, and the sensory signal reaches the central nervous system. However, motor output from the CNS to the bladder and urethral sphincter is blocked. Which response is most likely?
ⓐ. urine formation ceases despite intact renal filtration and tubular function
ⓑ. bladder filling is not sensed, although coordinated emptying remains intact
ⓒ. bladder filling is sensed, but motor coordination of emptying fails
ⓓ. glucose appears in urine despite normal proximal tubular reabsorption
Correct Answer: bladder filling is sensed, but motor coordination of emptying fails
Explanation: The intact receptors and sensory pathway allow bladder filling to be detected and conveyed to the central nervous system. The lesion affects the outgoing motor limb. Bladder smooth muscle does not receive the normal contraction signal, and the urethral sphincter does not receive the coordinated relaxation signal required for effective micturition. Sensation and motor execution are distinct stages of the reflex. Kidney filtration and tubular urine formation may continue, causing urine to accumulate despite impaired emptying. Glycosuria arises from metabolic or renal glucose-handling abnormalities rather than failure of bladder motor output. The case localises dysfunction by separating receptor activity, sensory transmission, central processing and motor response. Preserved sensing with blocked motor output localises the defect after central detection but before the coordinated bladder-and-sphincter response. The defect lies in the efferent limb of the reflex, so awareness of filling cannot produce the coordinated contraction-relaxation pattern required for release.
413. During intense metabolic activity in a hot environment, carbon-dioxide production rises and sweating increases. Kidney and liver functions remain normal. Which interpretation correctly integrates the excretory responses?
ⓐ. Lungs remove extra carbon dioxide; sweat cools and removes water, salts and trace wastes
ⓑ. Lungs remove extra carbon dioxide and most urea; sweat cools but has no excretory role
ⓒ. Sweat removes most extra carbon dioxide; lungs eliminate water, salts and small urea amounts
ⓓ. Kidneys become the main route for extra carbon dioxide; sweat replaces renal urea excretion
Correct Answer: Lungs remove extra carbon dioxide; sweat cools and removes water, salts and trace wastes
Explanation: Increased cellular activity generates more carbon dioxide, whose principal excretory route is through pulmonary ventilation. A hot environment also increases the need for surface cooling, so sweat production rises. Sweat consists mainly of water and sodium chloride and may carry small amounts of urea and lactic acid, giving it a secondary excretory role. Its primary function remains thermoregulation through evaporation. Normal kidney function continues to dominate nitrogenous-waste and ionic regulation, while the liver continues transferring selected pigments, cholesterol, altered hormones and drugs into bile. The case illustrates simultaneous cooperation among organ-specific routes without making them interchangeable. Each organ handles a characteristic part of the total excretory and homeostatic load.
414. A tracer crosses the glomerular capillary endothelium but is retained before it can reach the podocyte filtration slits. Which component of the filtration barrier most directly accounts for its retention?
ⓐ. Afferent-arteriole lumen before the glomerular capillaries
ⓑ. Bowman’s-capsule lumen beyond the filtration slits
ⓒ. Basement membrane between endothelium and podocytes
ⓓ. Proximal-tubule brush border after Bowman’s capsule
Correct Answer: Basement membrane between endothelium and podocytes
Explanation: Fluid moving from glomerular blood toward Bowman’s lumen first crosses the capillary endothelium, then the intervening basement membrane and finally the podocyte layer with its filtration slits. The tracer has already passed the endothelial component but has not reached the podocytes, placing the site of retention in the middle barrier layer. The basement membrane is the component most directly implicated by the stated position. Bowman’s lumen lies beyond all three filtration layers, while the PCT brush border acts later on filtrate that has already entered the renal tubule. The afferent-arteriole lumen is upstream of the glomerular capillaries. This spatial inference uses the order of the filtration barrier to identify the stopping point without relying on an unseen diagram or confusing glomerular selectivity with tubular reabsorption. Its position after the endothelium but before podocyte slits places the retained tracer at the basement membrane, the middle filtration layer.
415. Podocyte filtration slits are experimentally widened, while the glomerular endothelium, basement membrane and glomerular pressure remain normal. Which observation is most likely?
ⓐ. Increased passage of plasma proteins, while most blood cells remain excluded
ⓑ. Complete cessation of filtrate formation despite normal pressure
ⓒ. Increased passage of blood cells but complete retention of small solutes
ⓓ. Conversion of glomerular filtrate into concentrated urine inside Bowman’s capsule
Correct Answer: Increased passage of plasma proteins, while most blood cells remain excluded
Explanation: Podocytes form the outer epithelial layer of the glomerular filtration barrier, and spaces between their processes create fine filtration slits. Widening these slits weakens the final size-selective component of the barrier and can increase passage of proteins that are normally retained. The upstream endothelium and basement membrane remain intact, so unrestricted movement of formed blood cells is not expected. Glomerular pressure is also maintained, allowing filtrate volume to continue rather than cease completely. Bowman’s capsule collects the initial ultrafiltrate but does not perform the extensive reabsorption and concentration required to produce final urine. The experiment identifies a selective barrier defect: abnormal protein passage can arise even when the driving pressure and other filtration layers are preserved. Protein-rich filtrate under these controlled conditions specifically points to reduced slit selectivity, not failure of filtration pressure.
416. Use the arrangement described below. A nephron tubule leaves Bowman’s capsule, forms a loop and returns to the cortex. Near its own renal corpuscle, region P of the returning tubule closely contacts vessel Q entering the glomerulus. Cells in both structures are modified and form a sensitive regulatory region. Which identification is correct?
ⓐ. P-PCT; Q-efferent arteriole; region-renal pelvis
ⓑ. P-collecting duct; Q-renal vein; region-hilum
ⓒ. P-ascending vasa recta; Q-afferent arteriole; region-calyx
ⓓ. P-DCT; Q-afferent arteriole; region-renal JGA
Correct Answer: P-DCT; Q-afferent arteriole; region-renal JGA
Explanation: After passing through the loop of Henle, tubular fluid enters the DCT in the cortex. A portion of this DCT comes into close contact with the afferent arteriole supplying the nephron’s own glomerulus. Cellular modifications at this contact form the juxtaglomerular apparatus. The JGA senses changes related to glomerular blood flow, pressure and GFR and can initiate renin release when filtration falls. The efferent arteriole leaves rather than enters the glomerulus, while collecting ducts receive fluid farther downstream from many nephrons. Calyces, the renal pelvis and the hilum are larger kidney structures unrelated to this specialised microscopic contact. The complete identification depends on tubular position, vascular direction and regulatory function. The pathway runs from reduced renal perfusion to renin release, angiotensin II formation, aldosterone action and sodium-water retention. This contact is the juxtaglomerular apparatus, where a returning distal segment is anatomically linked to the vessel controlling glomerular inflow.
417. A nephron has an intact glomerulus, normal afferent blood supply and a structurally normal DCT. However, the modified contact region between the DCT and afferent arteriole is destroyed. When GFR falls, which response is most directly impaired?
ⓐ. Formation of filtration slits by podocytes
ⓑ. Renin release triggered by reduced filtration at the JGA
ⓒ. Water movement from the descending limb into the medulla
ⓓ. Passage of urine from the renal pelvis into the ureter
Correct Answer: Renin release triggered by reduced filtration at the JGA
Explanation: The destroyed contact region is the juxtaglomerular apparatus. Its modified cells connect information about reduced glomerular flow or GFR with release of renin. Loss of this region can impair initiation of the renin-angiotensin response even though the glomerulus, afferent blood supply and DCT remain individually present. Podocyte slit formation belongs to Bowman’s-capsule epithelium, while descending-limb water movement depends on medullary osmotic conditions. Urine transport from the renal pelvis into the ureter is a macroscopic drainage function. The lesion specifically disconnects a renal sensor from its hormonal output. A fall in GFR may persist without the normal pressure-supporting feedback that would otherwise promote angiotensin-II formation and aldosterone-related sodium-water recovery. The preserved structures narrow the defect to this feedback link.
418. Two healthy kidneys together contain nearly \(2.0\) million nephrons. One kidney is removed, and \(20\%\) of the nephrons in the remaining kidney subsequently become non-functional. Approximately how many functional nephrons remain?
ⓐ. \(0.20\) million
ⓑ. \(0.80\) million
ⓒ. \(1.20\) million
ⓓ. \(1.60\) million
Correct Answer: \(0.80\) million
Explanation: Each kidney contains nearly \(1.0\) million nephrons. Removal of one kidney leaves approximately \(1.0\) million nephrons in the remaining organ. If \(20\%\) of these become non-functional, the functional fraction is \(80\%\). The remaining number is \(1.0\times0.80=0.80\) million nephrons. The calculation does not imply that GFR or urine output must fall in exactly the same proportion, since functioning nephrons and regulatory mechanisms may alter their activity. It simply estimates the surviving functional-unit count under the stated conditions. The problem also reinforces that each kidney contains many independent nephrons rather than one large filtration unit, and removal of one kidney does not remove every nephron from the body. The percentage loss is applied only after removal has reduced the starting population to one million. Using twenty percent of the original two-million total would incorrectly count nephrons in the kidney that is no longer present.
419. Two kidneys have equal total nephron numbers and similar GFR values.
| Kidney | Short-loop nephrons | Long-loop nephrons with developed vasa recta | Response to water deprivation |
|---|
| P | Predominant | Relatively few | Moderate concentration |
| Q | Relatively few | Predominant | Marked concentration |
Which explanation best accounts for the difference?
ⓐ. P filters less because its short-loop nephrons lack functional glomeruli
ⓑ. Q produces more ADH within long loops and stores it in vasa recta
ⓒ. P lacks collecting ducts, so its cortical nephrons cannot deliver fluid
ⓓ. Q has greater counter-current capacity from long loops and vasa recta
Correct Answer: Q has greater counter-current capacity from long loops and vasa recta
Explanation: Long-loop juxtamedullary nephrons extend deep into the medulla and are accompanied by well-developed vasa recta. Their arrangement supports generation and preservation of a steep corticomedullary osmotic gradient. During water deprivation, collecting ducts can use this gradient under ADH influence to recover more water and produce highly concentrated urine. Short-loop cortical nephrons still possess glomeruli and collecting-duct connections, so their predominance does not abolish filtration or urine formation. Long loops do not produce ADH; the hormone is released from the neurohypophysis under hypothalamic control. Since total nephron number and GFR are similar, the contrasting concentrating ability is best explained by nephron type and medullary vascular architecture rather than by a difference in initial filtration. Kidney Q therefore has the stronger anatomical basis for concentrating urine and conserving water, even though both kidneys initially filter comparable amounts of fluid. The difference follows from long-loop and vasa-recta architecture, not from a higher GFR or ADH production by the nephron.
420. The brush border of PCT epithelial cells is selectively flattened without killing the cells or altering GFR. Which immediate combination is most likely?
ⓐ. Reduced surface, lower nutrient-ion recovery and increased downstream solute delivery
ⓑ. Increased filtration-slit area and passage of blood cells into Bowman’s capsule
ⓒ. Complete loss of ascending-limb electrolyte transport with stronger medullary osmolarity
ⓓ. Increased bladder contraction and reduced urethral-sphincter relaxation
Correct Answer: Reduced surface, lower nutrient-ion recovery and increased downstream solute delivery
Explanation: The PCT brush border consists of numerous microvillus-like projections that greatly expand the surface available for transport. Flattening this border reduces epithelial exchange area while leaving glomerular filtration intact. Recovery of glucose, amino acids, electrolytes and water is expected to decline, and a larger load of these substances reaches downstream nephron segments. The lesion does not alter podocyte filtration slits, which belong to Bowman’s capsule, or directly damage the ascending limb. Bladder and sphincter activity occur far downstream and are controlled through micturition pathways rather than PCT surface architecture. The prediction connects a microscopic structural adaptation with the PCT’s role in bulk reabsorption. An intact cell with greatly reduced surface area can remain alive yet perform transport less efficiently. Loss of apical surface area decreases contact with filtrate and transporter capacity, while preserved GFR confirms that initial filtration remains unaffected.
