Excretory System — Set 3

Correct Answer: B. Renal artery

• **Renal artery** = The renal arteries are paired branches that arise directly from the abdominal aorta at the level of the first and second lumbar vertebrae, delivering oxygenated blood to each kidney for filtration. • **High flow rate** — The kidneys receive approximately 20–25% of the total cardiac output (about 1.2 L/min) despite comprising less than 0.5% of body weight — reflecting the high metabolic demands of filtration. • 💡 Option A (Carotid artery) is wrong because the carotid arteries supply oxygenated blood to the head, brain, and neck; Option C (Pulmonary artery) is wrong because the pulmonary artery carries deoxygenated blood from the right ventricle to the lungs — it is the only artery carrying deoxygenated blood; Option D (Hepatic artery) is wrong because the hepatic artery supplies oxygenated blood specifically to the liver.

Correct Answer: C. Nephrolithiasis

• **Nephrolithiasis** = Nephrolithiasis (renal calculi or kidney stones) refers to the formation of hard, crystalline mineral deposits within the kidneys when urine becomes supersaturated with salts such as calcium oxalate, calcium phosphate, or uric acid. • **Common cause** — Inadequate fluid intake, a diet rich in oxalate or purines, and certain metabolic disorders all increase stone risk; small stones may pass spontaneously but large ones require lithotripsy or surgery. • 💡 Option A (Urethritis) is wrong because urethritis is inflammation of the urethra, usually caused by bacterial or sexually transmitted infection; Option B (Nephritis) is wrong because nephritis is a broader term for kidney inflammation (e.g., glomerulonephritis or pyelonephritis), not specifically stones; Option D (Cystitis) is wrong because cystitis is inflammation of the urinary bladder, most often caused by a bacterial infection.

Correct Answer: C. 180 Liters

• **180 Liters** = The kidneys produce about 180 L of glomerular filtrate per day (Glomerular Filtration Rate ≈ 125 mL/min); of this, 99% is reabsorbed, leaving only about 1.5–2 L as final urine. • **Efficiency** — Each litre of blood is filtered roughly 60 times a day, ensuring metabolic wastes are efficiently cleared even as nutrients and vital substances are selectively retained. • 💡 Option A (5 Liters) is wrong — 5 L is only the total blood volume in the body, which circulates through the kidneys multiple times a day; Option B (500 Liters) is wrong — this vastly overestimates the GFR; Option D (50 Liters) is wrong — 50 L is only about a quarter of the actual daily filtration volume.

Correct Answer: B. Cerebral Cortex

• **Cerebral Cortex** = The cerebral cortex (specifically the prefrontal cortex and anterior cingulate gyrus) provides voluntary control over the external urethral sphincter, allowing a person to consciously delay or initiate urination despite the involuntary micturition reflex. • **Higher override** — Toilet training in children is essentially the process of the cerebral cortex learning to suppress the sacral micturition reflex until a socially appropriate time. • 💡 Option A (Hypothalamus) is wrong because the hypothalamus regulates hormones (including ADH) and homeostasis, but does not directly control the voluntary sphincter; Option C (Cerebellum) is wrong because the cerebellum coordinates balance and fine motor movements, not sphincter control; Option D (Medulla oblongata) is wrong because the medulla controls autonomic functions like heart rate and breathing — the basic micturition reflex arc is at the sacral spinal cord level, not the medulla.

Correct Answer: D. It increases

• **It increases** = Excessive sweating causes significant fluid and electrolyte loss, which raises blood osmolality (blood becomes more concentrated); osmoreceptors in the hypothalamus detect this and stimulate the posterior pituitary to release more ADH. • **Compensatory response** — Higher ADH levels increase water reabsorption in the collecting ducts, producing darker and more concentrated urine to conserve the remaining body water. • 💡 Option A (It remains the same) is wrong because the body responds dynamically to changes in blood osmolality — ADH levels always adjust in response; Option B (It disappears) is wrong because ADH is never completely absent under physiological stress conditions; Option C (It decreases) is wrong because a decrease in ADH would cause more water loss through urine, worsening dehydration — exactly the opposite of what the body needs.

Correct Answer: A. Glomerulonephritis

• **Glomerulonephritis** = Glomerulonephritis is the inflammation of the glomerular capillaries, impairing their filtration function — it often presents with haematuria (blood in urine), proteinuria (protein in urine), reduced urine output, and oedema. • **Common cause** — It is frequently triggered by an abnormal immune response, such as post-streptococcal glomerulonephritis following a throat infection, where antibody-antigen complexes deposit in the glomeruli. • 💡 Option B (Pyelonephritis) is wrong because pyelonephritis is a bacterial infection that inflames the renal pelvis and kidney parenchyma, not specifically the glomeruli; Option C (Cystitis) is wrong because cystitis is inflammation confined to the urinary bladder; Option D (Uremia) is wrong because uremia is the toxic condition resulting from an accumulation of urea and other nitrogenous wastes in the blood due to kidney failure, not the same as glomerular inflammation.

Correct Answer: D. Calcium oxalate

• **Calcium oxalate** = Calcium oxalate is the most common component of kidney stones, accounting for about 75–80% of all cases — it crystallises when urinary calcium and oxalate concentrations are too high relative to the inhibitors normally present in urine. • **Dietary link** — Foods high in oxalate (spinach, nuts, chocolate) combined with low fluid intake significantly increase calcium oxalate stone risk; citrate in urine naturally inhibits crystal formation. • 💡 Option A (Iron oxide) is wrong because iron oxide is rust and is not a constituent of kidney stones in humans; Option B (Sodium chloride) is wrong because while high sodium intake increases urinary calcium excretion (indirectly raising stone risk), sodium chloride itself does not precipitate to form kidney stones; Option C (Magnesium sulfate) is wrong because magnesium actually inhibits stone formation — magnesium ammonium phosphate (struvite) stones are far less common and occur mainly with urinary infections.

Correct Answer: A. Proximal Convoluted Tubule

• **Proximal Convoluted Tubule (PCT)** = The PCT reabsorbs approximately 65–70% of filtered water, virtually 100% of filtered glucose and amino acids, and about 65–70% of sodium and chloride — all driven by active Na⁺/K⁺-ATPase pumps in the epithelial cells. • **Brush border** — The PCT cells have a dense brush border of microvilli that massively increases the surface area for efficient reabsorption of nutrients back into the peritubular capillaries. • 💡 Option B (Collecting duct) is wrong because the collecting duct primarily fine-tunes water reabsorption (regulated by ADH) and acid-base balance, not bulk nutrient recovery; Option C (Henle's loop) is wrong because the loop of Henle is specialised for establishing the medullary osmotic gradient, not for nutrient reabsorption; Option D (Distal tubule) is wrong because the distal convoluted tubule handles fine adjustments of sodium, potassium, and acid-base balance under hormonal control (aldosterone, ADH).

Correct Answer: A. Maintaining medullary gradient

• **Maintaining medullary gradient** = Vasa recta are long, hairpin-shaped capillaries that run parallel to the loop of Henle — acting as counter-current exchangers, they absorb solutes and water from the medullary interstitium as they descend and return them as they ascend, preserving the hypertonic gradient without washing it away. • **Nutrient supply** — They also deliver oxygen and nutrients to the metabolically active medullary cells while removing the products of their metabolism. • 💡 Option B (Blood filtration) is wrong because blood filtration occurs exclusively at the glomerulus — vasa recta are peritubular capillaries that recover reabsorbed materials rather than filter; Option C (Urea synthesis) is wrong because urea is synthesised in the liver through the urea cycle, not in the kidney vasculature; Option D (Urine storage) is wrong because urine is stored in the urinary bladder — the kidney's collecting ducts channel urine to the renal pelvis.

Correct Answer: C. Birds

• **Birds** = Birds (and reptiles) are uricotelic — they excrete nitrogenous waste as semi-solid uric acid paste, which requires very little water to excrete, making them highly water-efficient compared with ureotelic mammals. • **Evolutionary advantage** — This adaptation is critical for birds in flight and for embryos developing inside eggs, where liquid waste products cannot be expelled. • 💡 Option A (Amphibians) is wrong because amphibians are largely ammonotelic in water (excreting ammonia) and ureotelic on land, depending on their environment; Option B (Fish) is wrong because most bony fish are ammonotelic — they excrete ammonia directly across their gills into the surrounding water; Option D (Mammals) is wrong because mammals, including humans, are ureotelic — they convert ammonia to urea in the liver and excrete it dissolved in urine.