The hormone melatonin which regulates sleep is secreted by which gland?

Correct Answer: A

Rationale: Melatonin, regulating sleep, is secreted by the pineal gland, not listed; however, pituitary (A) is incorrect pineal is intended but missing. Corpus luteum makes progesterone, progesterone's a hormone, melatonin's not a gland. Assuming error, pineal aligns with melatonin's role. This circadian function distinguishes it, key to sleep patterns, contrasting with reproductive or regulatory glands.

Correct Answer: C

Rationale: T4's action is slower than T3's converted to T3 for potency, acting via nuclear receptors over hours. Iodide is actively transported into colloid for synthesis. MIT/DIT (intermediates) aren't secreted only T3/T4. TBG, not albumin, binds most T4 albumin has capacity but lower affinity. Slower T4 action distinguishes it, key to thyroid kinetics, unlike transport, secretion, or binding truths.

Correct Answer: C

Rationale: Thyroid hormones (T3/T4) boost Na/K ATPase (metabolism), nitrogen excretion (catabolism), and α-myosin chains (cardiac efficiency), but don't directly increase catecholamine levels they sensitize tissues to them via β-receptors. Catecholamine production is adrenal, not thyroid-driven. Exclusion of catecholamine increase distinguishes it, key to thyroid's calorigenic scope, unlike metabolic or cardiac effects.

Correct Answer: D

Rationale: High plasma Ca²⁺ reduces 1,25-dihydroxycholecalciferol (active vitamin D) via feedback, inhibiting renal 1α-hydroxylase balances absorption. ~99% filtered Ca is reabsorbed, not 60%. GIT Ca absorption is active (vitamin D-dependent), not mainly passive. Ca binding is proportional to protein levels (e.g., albumin), not inverse. Vitamin D reduction distinguishes it, key to Ca homeostasis, unlike reabsorption, absorption, or binding errors.

Correct Answer: B

Rationale: Somatostatin inhibits insulin secretion from β-cells, reducing glucose uptake regulatory role. Insulin promotes protein synthesis, not catabolism anabolic. Phenytoin doesn't notably stimulate insulin glucose metabolism does. Insulin increases K⁺ uptake with glucose, not decreases. Somatostatin's inhibition distinguishes it, key to pancreatic regulation, unlike anabolic, drug, or K⁺ errors.