Most of the endocrine system is regulated by:

Correct Answer: A

Rationale: Negative feedback, where hormone levels inhibit further release (e.g., T4 suppressing TSH), stabilizes the endocrine system. Positive feedback (e.g., oxytocin in labor) is rare, and complexes describe mechanisms, not regulation. Negative feedback's prevalence distinguishes endocrine control, critical for homeostasis, contrasting with amplifying loops.

Correct Answer: B

Rationale: Adrenaline (epinephrine) is secreted by the adrenal glands' medulla, atop the kidneys, triggering the fight-or-flight response by elevating heart rate and glucose availability. The thyroid produces thyroxine for metabolism, the pineal melatonin for sleep neither secrete adrenaline. 'More than one' is invalid as adrenaline is adrenal-specific. This gland's role in acute stress responses distinguishes it, key to survival mechanisms, contrasting with metabolic or circadian regulators.

Correct Answer: B

Rationale: Adrenaline, from the adrenal medulla, prepares the body for emergencies by accelerating heart rate, dilating airways, and mobilizing glucose 'fight or flight.' Pineal (melatonin) aids sleep, pituitary (GH, ACTH) regulates long-term, pancreas (insulin) manages sugar not rapid action. Adrenal's acute stress response distinguishes it, vital for survival, contrasting with metabolic or sleep hormones.

Correct Answer: C

Rationale: Angiotensinogen, synthesized by the liver, is cleaved by kidney-derived renin to angiotensin I, driving blood pressure regulation via angiotensin II. Prorenin is largely inactive, not 50% active. Propranolol (β-blocker) reduces renin by blocking sympathetic stimulation, not increasing it. Angiotensin I is inactive, not a vasodilator angiotensin II vasoconstricts. Liver synthesis distinguishes angiotensinogen's role, essential for RAAS activation, unlike precursor activity, drug effects, or vasodilation errors.

Correct Answer: C

Rationale: Thyroxine-binding globulin (TBG) binds most circulating T4 (~70%), stabilizing it in plasma for transport high affinity defines it. Prealbumin (transthyretin) binds less (~15-20%). 'Tramothynetim' is a typo, likely irrelevant. A2 globulin isn't a thyroid binder albumin binds some (~10%). TBG's dominance distinguishes it, critical for T4's bioavailability, unlike minor or erroneous binders.