Calcium level in the blood is regulated by the:

Correct Answer: A

Rationale: The correct answer is A: Parathyroid and thyroid. The parathyroid glands secrete parathyroid hormone, which increases blood calcium levels by promoting calcium release from bones and increasing calcium absorption in the intestines. The thyroid gland produces calcitonin, which lowers blood calcium levels by inhibiting bone breakdown and increasing calcium excretion by the kidneys. The adrenal medulla and pancreas do not play a direct role in regulating calcium levels. Testes are not involved in calcium regulation. The thymus is responsible for developing T-cells and does not have a role in calcium regulation.

Correct Answer: D

Rationale: The correct answer is D. During pregnancy, high levels of estrogen and progesterone inhibit lactation by suppressing the release of prolactin, the hormone responsible for milk production. Estrogen and progesterone maintain the pregnancy and prevent premature labor, so the body prioritizes their functions over lactation. Choices A, B, and C are incorrect because low blood prolactin levels, human placental lactogen levels, and fetal adrenal gland's estriol production are not directly related to the suppression of lactation during pregnancy.

Correct Answer: A

Rationale: The correct answer is A because thyroid hormone is a lipid-soluble hormone that can diffuse across the cell membrane and bind to intracellular receptors within the nuclei. This binding activates gene transcription, leading to various metabolic effects. Choice B is incorrect because thyroid hormone actually has a significant impact on body metabolism. Choice C is incorrect as thyroid hormone does not directly affect blood vessel adrenergic receptors or blood pressure. Choice D is incorrect as thyroid hormone actually acts to increase basal metabolic rate, not decrease it.

Correct Answer: D

Rationale: The correct answer is D: humoral stimulation. This mechanism directly responds to changing blood levels of ions and nutrients. Humoral stimulation refers to the regulation of hormone secretion in response to changes in blood levels of various substances like ions and nutrients. It is a feedback mechanism that helps maintain homeostasis by adjusting hormone levels based on the body's needs. A: Rapid oxidation of carbohydrates is not related to endocrine control systems responding to changing blood levels of ions and nutrients. B: Catabolic inhibition does not directly respond to changing blood levels of ions and nutrients. C: Protein synthesis is not a mechanism that directly responds to changing blood levels of ions and nutrients.

Correct Answer: D

Rationale: The correct answer is D: Parathyroid hormone is the single most important regulator of calcium levels in the blood. Parathyroid hormone plays a crucial role in maintaining calcium homeostasis by stimulating bone resorption, increasing calcium reabsorption in the kidneys, and activating vitamin D to enhance calcium absorption in the intestines. It acts to raise blood calcium levels when they are low. Explanation for other choices: A: Increased calcitonin levels actually lower blood calcium levels by promoting calcium deposition in bones. B: High calcium levels do not cause bone resorption; it is actually the opposite as mentioned in choice D. C: Parathyroid hormone does not increase osteoblast activity; it primarily affects osteoclasts to release calcium from bones.