Which mechanism is primarily responsible for maintaining hormone levels within a narrow range?

Correct Answer: A

Rationale: The correct answer is A: negative feedback mechanisms. Negative feedback mechanisms help maintain hormone levels within a narrow range by inhibiting further hormone production once a certain level is reached. When hormone levels rise, negative feedback signals the body to decrease hormone production, preventing excessive levels. Positive feedback mechanisms, on the other hand, amplify hormone production. Hormone-receptor complexes and hormone-gene complexes are not directly responsible for maintaining hormone levels within a narrow range but rather play roles in hormone signaling and gene expression, respectively.

Correct Answer: C

Rationale: Plasma osmolality is primarily regulated by the concentration of solutes in the blood. Vasopressin, also known as antidiuretic hormone (ADH), plays a key role in regulating water reabsorption in the kidneys, thus affecting plasma osmolality. When plasma osmolality increases, vasopressin is released to increase water reabsorption, leading to a decrease in plasma osmolality. Progesterone, cortisol, and aldosterone do not directly influence water reabsorption and therefore do not have as significant an impact on plasma osmolality as vasopressin.

Correct Answer: D

Rationale: The correct answer is D because steroid hormones bind to a receptor inside the target cell, are made from cholesterol, and cause the cell to make new proteins. This is known as the mechanism of steroid hormone action. Choice A is correct as steroid hormones need to bind to intracellular receptors to exert their effects. Choice B is also correct as steroid hormones are derived from cholesterol. Choice C is correct as one of the main actions of steroid hormones is to regulate gene expression and induce the synthesis of new proteins within the cell. Therefore, all of these statements are true regarding steroid hormones, making choice D the correct answer.

Correct Answer: D

Rationale: The correct answer is D: hyperthyroidism. Nervousness, increased body temperature, and increased blood pressure are classic symptoms of hyperthyroidism. This condition is characterized by an overactive thyroid gland, leading to an excess production of thyroid hormones. These hormones can cause an increase in metabolism, leading to symptoms such as nervousness, heat intolerance, and elevated heart rate. Diabetes mellitus (A) is not associated with increased body temperature or blood pressure. Hypoglycemia (B) is characterized by low blood sugar levels, leading to symptoms such as dizziness and confusion, not increased body temperature. Hypothyroidism (C) is characterized by an underactive thyroid gland, leading to symptoms such as fatigue, weight gain, and cold intolerance, not increased body temperature or blood pressure.

Correct Answer: D

Rationale: The correct answer is D: low levels of calcium in the blood. When calcium levels drop, the parathyroid gland releases parathyroid hormone (PTH) to increase calcium levels in the blood. This is a negative feedback mechanism to maintain calcium homeostasis. A: TSH from the posterior pituitary gland is incorrect because TSH is involved in regulating thyroid function, not PTH release. B: High levels of calcium in the blood would not stimulate PTH release as the body aims to lower, not raise, calcium levels. C: Calcitonin from the anterior pituitary gland is incorrect because calcitonin is released by the thyroid gland and works to lower blood calcium levels, opposing the action of PTH.