Calcitonin and parathyroid hormone:

Correct Answer: D

Rationale: Correct Answer: D Rationale: Calcitonin and parathyroid hormone regulate plasma calcium levels. Calcitonin decreases calcium levels by inhibiting bone breakdown, while parathyroid hormone increases calcium levels by stimulating bone breakdown and increasing calcium reabsorption. Therefore, both hormones work together to maintain calcium homeostasis. The correct answer, choice D, highlights that both hormones control plasma calcium levels. Summary of Other Choices: A: Incorrect. Calcitonin is secreted by the thyroid gland, not the parathyroid glands. B: Incorrect. Calcitonin decreases plasma calcium levels, while parathyroid hormone increases plasma calcium levels. C: Incorrect. Parathyroid hormone increases plasma calcium levels, whereas calcitonin decreases plasma calcium levels.

Correct Answer: B

Rationale: The correct answer is B: A benign tumor of the adrenal medulla is called a pheochromocytoma. Pheochromocytomas are tumors that arise from the adrenal medulla, leading to overproduction of catecholamines like adrenaline and noradrenaline. This excess secretion can result in symptoms such as high blood pressure, palpitations, and sweating. Choices A and C are incorrect because a benign tumor of the adrenal medulla typically causes high blood pressure, not low blood pressure or Addison's disease. Choice D is incorrect because excess cortisol secretion is associated with conditions like Cushing's syndrome, not pheochromocytoma.

Correct Answer: B

Rationale: The correct answer is B because as plasma levels of calcium decrease, the parathyroid glands secrete an osteoclastic hormone called parathyroid hormone (PTH). PTH acts to increase calcium levels by stimulating the release of calcium from bones via osteoclasts. Insulin secretion (choice A) is not directly related to calcium levels. The kidneys excreting calcium and phosphorus (choice C) would further decrease calcium levels. Calcitonin (choice D) actually decreases blood calcium levels by inhibiting bone breakdown, so it is not secreted when calcium levels decrease.

Correct Answer: B

Rationale: The correct answer is B. The pituitary gland does not directly control the parathyroid gland secretion of PTH. The pituitary gland is responsible for secreting hormones that regulate other endocrine glands, such as adrenal cortex (A), thyroid gland (C), and ovaries (D). In contrast, the parathyroid gland is primarily regulated by the calcium levels in the blood, not directly by the pituitary gland. Therefore, choices A, C, and D are incorrect as they are all glands whose secretions are directly controlled by the pituitary gland.

Correct Answer: C

Rationale: Rationale: Myxedema, Graves' disease, and cretinism are all disorders directly related to the thyroid gland, involving either hypo or hyperactivity. Myxedema and cretinism are hypothyroid disorders, while Graves' disease is a hyperthyroid disorder. Thus, the correct answer is C. Choice A is incorrect as Graves' disease is a hyperthyroid disorder. Choice B is incorrect as these disorders are not solely due to a deficiency of TSH. Choice D is incorrect as treatment can vary depending on the specific disorder and may not always involve T3 and/or T4.