Which is TRUE?

Correct Answer: B

Rationale: PTH inhibits phosphate reabsorption in the PCT to increase blood Ca2+. It enhances Ca2+ reabsorption in the DCT, not PCT, and affects osteoblasts, not osteoclasts directly. Calcitonin and oestrogens inhibit osteoclasts.

Correct Answer: A

Rationale: Uric acid is a product of purine (e.g., adenine, guanine) catabolism, not pyrimidines, glutamine, or urea.

Correct Answer: B

Rationale: Cholera toxin increases cAMP, opening Cl^- channels, causing diarrhea. Na^+/glucose co-transport remains effective (basis for ORS).

Correct Answer: D

Rationale: Acetylcholine stimulates glucagon via vagal input; somatostatin inhibits, FFAs and α-agonists can stimulate, GABA has less direct effect.

Correct Answer: A

Rationale: Aldosterone is classified as a mineralocorticoid. It promotes conservation of water by acting on the kidneys to retain sodium in exchange for potassium, which is excreted in the urine.