Phosphate buffer system does not have much effect as intracellular fluid buffer. However, it's a major extracellular buffer and important in renal tubular fluid?

Correct Answer: B

Rationale: The phosphate buffer system (HPO₄²⁻/H₂PO₄⁻) is a major intracellular buffer due to high phosphate in cells, not less effective (statement's first part is false). It's also significant extracellularly in renal tubular fluid, titrating H⁺ for excretion, but not ‘major' compared to bicarbonate in plasma. The mixed accuracy makes B (False) correct.

Correct Answer: D

Rationale: Urea, a waste product from protein breakdown, is filtered and excreted by the kidneys. Regular monitoring (e.g., blood urea nitrogen, BUN) evaluates kidney health (D), as impaired function raises levels (e.g., renal failure). Liver disorders (A) affect urea production, metabolic health (B) is vague, and deficiencies (C) aren't a concern, making D correct.

Correct Answer: D

Rationale: The proximal tubule reabsorbs ~65% of filtrate iso-osmotically (A), PAH concentration rises as it's secreted (B is false), and glucose/bicarbonate are mostly reabsorbed (C). A and C are true—reabsorption is iso-osmotic, nutrients are reclaimed—while B is incorrect, making D (A and C are correct) the answer.

Correct Answer: B

Rationale: Rising pCO₂ (80 mmHg) from hypoventilation lowers blood pH (acidosis) rapidly. CSF pH falls faster as CO₂ diffuses quickly across the blood-brain barrier, outpacing blood buffering (e.g., HCO₃⁻). Blood pH lags due to systemic buffers, making B correct.

Correct Answer: C

Rationale: Micturition is a sacral reflex (S2-S4) (C), contracting the detrusor (not relaxing, A) and relaxing sphincters. The external sphincter relaxes, not contracts (B), during voiding, making C the correct process description.