A hormone is best described as:

Correct Answer: D

Rationale: A hormone is a chemical produced by an endocrine gland, released into the bloodstream, and transported to distant target cells where it regulates physiological processes, like insulin from the pancreas controlling blood glucose elsewhere. Duct transport fits exocrine glands (e.g., sweat), not hormones. Many effects apply to some hormones (e.g., cortisol), but it's not definitive specificity matters. Gland secretion is true but incomplete transport and action define hormones, not just origin. The full description captures hormones' endocrine nature, distinguishing them from local or ducted secretions, key to their systemic regulatory role.

Correct Answer: D

Rationale: Parathyroid hormone (PTH) targets kidneys (Ca²⁺ reabsorption), bones (Ca²⁺ release), and intestines (via vitamin D for Ca²⁺ uptake) to raise blood calcium. Muscles aren't direct targets PTH doesn't alter their function, though low calcium from PTH absence causes tetany indirectly. 'None' is incorrect PTH affects listed organs. Muscles' exclusion distinguishes them, key to PTH's calcium-focused action, unlike regulated sites.

Correct Answer: D

Rationale: The pancreas is both endocrine (islets secrete insulin/glucagon into blood) and exocrine (acinar cells release digestive enzymes via ducts). Adrenal, pituitary, and thyroid are purely endocrine, secreting hormones (e.g., cortisol, GH, thyroxine) into blood, no ducts. Pancreas' dual role regulating glucose and aiding digestion distinguishes it, vital for metabolic and digestive integration, unlike single-function glands.

Correct Answer: A

Rationale: Elevated ACTH from the anterior pituitary stimulates the adrenal cortex's zona fasciculata to release glucocorticoids (e.g., cortisol), regulating stress and metabolism. Mineralocorticoids (e.g., aldosterone) from zona glomerulosa respond to renin-angiotensin, not ACTH primarily. Epinephrine, from the adrenal medulla, isn't ACTH-driven sympathetic signals control it. Insulin, pancreatic, counters glucose, unrelated to ACTH. Glucocorticoids' ACTH dependency distinguishes them, key to the hypothalamic-pituitary-adrenal axis, unlike electrolyte, medullary, or glucose responses.

Correct Answer: B

Rationale: Paracrine signaling involves chemicals, like prostaglandins, affecting nearby cells, as in inflammation. Autocrine signals target the secreting cell itself (e.g., cancer cells), endocrine signals travel via blood to distant targets, and 'neuron' isn't a signaling type neurons use neurotransmitters. Paracrine's local action distinguishes it, key for short-range cellular communication, contrasting with self-directed or systemic signaling in physiological responses.