Which of the following structures functions as a countercurrent exchanger?

Correct Answer: D

Rationale: Vasa recta acts as a countercurrent exchanger maintains medullary gradient (e.g., loops with Loop of Henle). Afferent arterioles supply not exchange. Urethra drains no countercurrent. Trigone is bladder not exchanger. Vasa recta's exchange distinguishes it, critical for concentration, unlike supply or drainage structures.

Correct Answer: A

Rationale: Increased cardiac output boosts GFR e.g., more blood pressure (~55 mmHg) at glomerulus. Lower BP decreases GFR opposite. Less fluid reduces volume not GFR directly. Temperature affects skin not kidneys. Cardiac output's effect distinguishes it, key to filtration rate, unlike pressure drop or external factors.

Correct Answer: B

Rationale: Urea clearance = UV/P: U=18.2 mg/mL, V=2 mL/min, P=26 mg/100 mL = 0.26 mg/mL. (18.2 × 2) / 0.26 = 36.4 / 0.26 ≈ 70 mL/min e.g., partial reabsorption. 35 underestimates half off. 105/140 overstate exceed GFR. 70 distinguishes it, critical for clearance calc, unlike wrong values.

Correct Answer: A

Rationale: Angiotensin II stimulates zona glomerulosa e.g., aldosterone release for Na retention. JG cells secrete renin not AII. Urethra is exit not hormone. Macula densa is JGA not stimulator. Adrenal trigger distinguishes it, critical for RAAS, unlike secretion or structural errors.