Which of the following explains the suppression of lactation during pregnancy?

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: 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.

Correct Answer: D

Rationale: The correct answer is D: breaking down nutrients. Hormones are chemical messengers that regulate various physiological processes in the body. Hormones do not directly break down nutrients; instead, they regulate the processes involved in nutrient breakdown. A: maintaining homeostasis is a function of hormones as they help regulate internal balance. B: influencing metabolism is a function of hormones as they regulate energy production and utilization. C: influencing growth and development is a function of hormones as they play a crucial role in the growth and maturation of cells and tissues.

Correct Answer: B

Rationale: The correct answer is B: kidney and blood vessels. ADH, or antidiuretic hormone, acts on the kidney to regulate water reabsorption and maintain fluid balance in the body. It also acts on blood vessels to regulate blood pressure. The other choices are incorrect because ADH does not target the adrenal cortex, liver, pituitary gland, or thyroid gland specifically for its primary functions. ADH primarily acts on the kidney and blood vessels to regulate water balance and blood pressure in the body.