Sodium is mostly reabsorbed by:

Correct Answer: B

Rationale: Sodium (Na⁺) is mostly reabsorbed via primary active transport, driven by the Na⁺/K⁺-ATPase pump on the basolateral membrane of tubular cells, especially in the proximal convoluted tubule (PCT). This pump uses ATP to move Na⁺ out into the peritubular capillaries against its gradient, creating a low intracellular Na⁺ concentration. This facilitates Na⁺ entry from the lumen via secondary mechanisms, but the primary energy-dependent step defines the process, making B correct.

Correct Answer: C

Rationale: Potassium secretion in the cortical collecting tubule by principal cells is driven by aldosterone, which increases Na⁺ reabsorption and K⁺ secretion. Spironolactone, an aldosterone antagonist, inhibits this, reducing K⁺ secretion. High plasma K⁺ (A) and alkalosis (D) increase secretion, and diuretics like furosemide (B) indirectly enhance it, making C the correct inhibitor.

Correct Answer: B

Rationale: The fastest system controlling [H⁺] is CO₂ excretion by the lungs, acting within seconds to minutes via increased ventilation, reducing PCO₂ and thus H₂CO₃ (H⁺ source). Buffers (C) act instantly but don't eliminate H⁺, kidney H⁺ excretion (A) takes hours, and carbonic anhydrase (D) is a mechanism, not a system, making B correct.

Correct Answer: B

Rationale: Paternalism (B) occurs when others (e.g., family) assume decision-making authority, believing they know best, overriding patient autonomy. Justice (A) involves fairness, pragmatism (C) practicality, and veracity (D) truthfulness, none of which fit this assumption of control without patient input, making B correct.

Correct Answer: B

Rationale: Erythropoietin (EPO) release by kidney interstitial cells is triggered by low blood oxygen levels (B) (hypoxia), stimulating red blood cell production in bone marrow to improve oxygen delivery. High oxygen (A) or RBCs (C) suppress it, and COâ‚‚ (D) isn't the trigger, making B correct.