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A. Gated channels: Some signaling molecules influence ion channels, but hormones do not act only on gated channels - many act via receptors that trigger intracellular cascades.
B. Receptors: Hormones (like neurotransmitters) act only on cells that have specific receptors to which the hormone binds (membrane receptors for peptide hormones or intracellular receptors for lipophilic hormones). This is the correct answer.
C. Enzymes: Hormones can modulate enzymes inside cells, but they require receptors to target cells first; enzymes are not the general targeting mechanism.
D. Cofactors: Cofactors assist enzymatic reactions but are not the specific binding target that determines hormone action on particular cells.
E. Metabolic pathways: Hormones influence metabolic pathways, but they act via receptors on/inside target cells - metabolic pathways are downstream, not the specific binding site.

A. slowly; slowly; widespread: The nervous system responds quickly, not slowly, and its effects are generally more specific/localized than widespread.
B. quickly; quickly; specific: The nervous system reacts quickly to stimuli (milliseconds to seconds), adapts quickly (e.g., rapid onset/offset, habituation), and produces specific targeted effects via precise neuronal pathways - contrasts with the endocrine system which is slower to start, slower to adapt, and often produces more widespread effects.
C. slowly; quickly; specific: The nervous system does not react slowly; it reacts quickly.
D. quickly; slowly; specific: While the nervous system reacts quickly, it typically adapts quickly, not slowly; endocrine responses adapt more slowly.
E. quickly; quickly; widespread: The nervous system is quick, but its effects are usually specific/local, not widespread - endocrine signaling is the one usually producing widespread effects.

A. Kidneys: ADH (vasopressin) increases water reabsorption in the kidney’s collecting ducts, reducing urine volume and conserving body water.
B. Pancreas: The pancreas regulates blood sugar via insulin and glucagon; it is not the target of ADH.
C. Hypothalamus: ADH is synthesized in the hypothalamus (supraoptic & paraventricular nuclei), but the hypothalamus is not its main target organ.
D. Anterior pituitary: The anterior pituitary is regulated by hypothalamic releasing hormones, not ADH.
E. Adrenal gland: The adrenal gland produces corticosteroids and catecholamines, but ADH does not act directly on it.

A. Melatonin: Secreted by the pineal gland, melatonin regulates circadian rhythms and sleep-wake cycles.
B. Hepcidin: Hepcidin regulates iron metabolism, not circadian rhythm.
C. Inhibin: Inhibin regulates FSH secretion in the reproductive system.
D. Calcitonin: Calcitonin (thyroid hormone) lowers blood calcium levels; it has no role in circadian rhythm.
E. Melanin: Melanin is a pigment in skin/hair/eyes, not a hormone regulating circadian rhythm.

A. Antagonistic regulation: This refers to hormones with opposing effects (e.g., insulin vs glucagon), not pituitary regulation.
B. Down-regulation: Down-regulation occurs when target cells reduce receptor numbers in response to high hormone levels, not a feedback mechanism to the pituitary.
C. Positive feedback inhibition: Positive feedback is rare (e.g., oxytocin in childbirth) - most pituitary regulation occurs via negative feedback.
D. Up-regulation: Up-regulation increases receptor sensitivity but is not the primary mechanism of pituitary control.
E. Negative feedback inhibition: Most endocrine regulation occurs through negative feedback, where hormones from target organs (like thyroid hormone or cortisol) suppress pituitary hormone release.

A. thymus: The thymus secretes thymosins that influence T-cell maturation (immune function), not hormones that raise blood calcium - so it does not respond to hypocalcemia.
B. thyroid gland: The thyroid’s parafollicular (C) cells secrete calcitonin, which lowers blood calcium in response to hypercalcemia, not hypocalcemia.
C. pituitary gland: The pituitary releases many trophic hormones, but it does not directly secrete the primary hormone (PTH) that corrects hypocalcemia.
D. parathyroid glands: The parathyroid glands secrete parathyroid hormone (PTH) in response to low blood Ca²⁺; PTH raises serum calcium by acting on bone, kidney, and vitamin D activation.
E. pineal gland: The pineal gland secretes melatonin to regulate circadian rhythms, not calcium homeostasis.

A. adrenal medulla; epinephrine: The adrenal medulla secretes epinephrine (and norepinephrine) for fight-or-flight effects (heart rate, glycogenolysis); it is not the main hormone that promotes Na⁺ and water retention.
B. pancreas; cortisol: The pancreas secretes insulin and glucagon, not cortisol; cortisol is produced by the adrenal cortex and has limited mineralocorticoid activity but is not the principal Na⁺/water-retaining hormone.
C. kidneys: corticosterone: The kidneys do not secrete corticosterone; corticosterone is an adrenal steroid (minor mineralocorticoid in some species).
D. adrenal cortex, aldosterone: Aldosterone (zona glomerulosa of the adrenal cortex) increases Na⁺ reabsorption and water retention and promotes K⁺ excretion in the kidney.
E. thyroid: calcitonin: Calcitonin from the thyroid lowers blood Ca²⁺ and has no role in Na⁺/water retention.