The hormone whose action resembles stimulation through the sympathetic nervous system is:

Correct Answer: A

Rationale: Epinephrine, from the adrenal medulla, mimics sympathetic activation, raising heart rate, dilating pupils, and mobilizing glucose, akin to fight-or-flight. Cortisol, a glucocorticoid, manages chronic stress and metabolism, not acute sympathetic effects. Androgens (e.g., testosterone) drive sex traits, not sympathetic. Aldosterone regulates electrolytes, not mimicking neural responses. Epinephrine's rapid, catecholamine-driven action distinguishes it, critical for acute stress, unlike slower or unrelated hormones.

Correct Answer: C

Rationale: Negative feedback in the endocrine system adjusts hormone levels inversely: low T₃ and T₄ (thyroid hormones) signal the pituitary to increase TSH, stimulating thyroid output to restore levels. High T₃/T₄ would decrease TSH, not increase, opposing feedback's corrective nature. Low T₃/T₄ decreasing TSH would worsen deficiency, defying regulation. No effect ignores feedback entirely TSH rises to compensate. This inverse response exemplifies negative feedback, distinguishing it, essential for homeostasis, unlike positive or null reactions.

Correct Answer: B

Rationale: Intracellular hormone receptors, typically in the nucleus or cytoplasm, bind steroid or thyroid hormones, which cross membranes due to lipid solubility. Thyroid hormone (T3/T4) uses such receptors to regulate metabolism. Melatonin, growth hormone, and insulin act via membrane receptors, not intracellular ones. Pesticide binding could mimic or block thyroid hormone, disrupting metabolic balance, distinguishing it as the likely target, critical for endocrine disruption studies.

Correct Answer: C

Rationale: A goiter, thyroid enlargement, often results from excessive colloid (thyroglobulin storage) accumulation, typically from iodine deficiency or TSH overstimulation, impairing hormone synthesis. Pituitary enlargement or growth hormone excess (acromegaly) don't cause goiters, nor does follicle hypertrophy alone. Colloid buildup distinguishes goiter pathology, key to diagnosing thyroid dysfunction.

Correct Answer: B

Rationale: Melatonin production by pinealocytes rises in darkness and is inhibited by bright light, detected by retinal-suprachiasmatic pathways, suppressing synthesis. Declining light boosts it, serotonin is a precursor (not inhibitor), and pinealocyte activity drives production. Light's inhibitory effect distinguishes melatonin regulation, key to sleep timing, contrasting with darkness-driven increase.