Melatonin is an antioxidant that

Correct Answer: B

Rationale: Melatonin neutralizes free radicals, protecting cells from oxidative damage.

Correct Answer: C

Rationale: TRH stimulates TSH release, leading to thyroid hormone production.

Correct Answer: D

Rationale: Osteoblasts build bone matrix and differentiate into osteocytes.

Correct Answer: C

Rationale: Insulin promotes ketone uptake by muscles during glucose abundance, not ketogenesis (a glucagon effect). It increases amino acid uptake and protein synthesis, not catabolism.

Correct Answer: D

Rationale: Chronic low Ca2+ triggers secondary hyperparathyroidism. High Ca2+ suppresses PTH and doesn't cause tetany (low Ca2+ does). ~99% of Ca2+ is in bones, and infant turnover is high but not precisely 80%.