Osteitis fibrosa cystica is caused by __________.

Correct Answer: D

Rationale: Osteitis fibrosa cystica is caused by an increase in the parathyroid hormone (PTH). Elevated PTH levels lead to increased calcium release from bones, resulting in bone resorption and cyst formation. Other choices (A) calcitonin, (B) ADH, and (C) growth hormone do not directly impact calcium metabolism and bone resorption like PTH does.

Correct Answer: d

Rationale: Rationale for Correct Answer (D): 1. Ca2+ ions do not directly break down ATP into ADP + inorganic phosphate at the sarcomere. 2. This breakdown occurs through the enzymatic action of myosin ATPase. 3. Ca2+ ions primarily bind to troponin to initiate muscle contraction by moving tropomyosin. 4. Ca2+ ions help expose active sites on actin, facilitating cross-bridge formation. Summary of Incorrect Choices: A. Correct - Ca2+ binds to troponin to move tropomyosin, not to uncover it. B. Correct - Ca2+ aids in exposing active sites on actin, not troponin. C. Correct - Ca2+ is stored in the sarcoplasmic reticulum, but it is released into the cytoplasm to trigger muscle contraction, not stored in the terminal cisternae.

Correct Answer: A

Rationale: Step-by-step rationale for choice A being correct: 1. ADH and oxytocin are peptide hormones. 2. They are synthesized in the hypothalamus. 3. They are transported and stored in the neurohypophysis. 4. Upon stimulation, they are released from the neurohypophysis. 5. Therefore, both ADH and oxytocin are indeed secreted by the neurohypophysis. Summary of why other choices are incorrect: B: ADH and oxytocin are not mineralocorticoids; they regulate water balance and uterine contractions, respectively. C: Islets of Langerhans secrete insulin and glucagon, not ADH or oxytocin. D: ADH promotes water reabsorption in the kidneys, opposite to excreting Na+ and water.

Correct Answer: D

Rationale: Rationale: 1. Insulin, cortisol, thyroxine, and oxytocin are all hormones produced by various glands in the body. 2. Insulin is produced by the pancreas, cortisol by the adrenal glands, thyroxine by the thyroid gland, and oxytocin by the hypothalamus and released by the pituitary gland. 3. Each of these hormones plays a different role in the body, such as regulating blood glucose levels, metabolism, stress response, and reproductive functions. 4. Therefore, the correct answer is D: they are all hormones. Summary: A: Incorrect. Hypothalamic-releasing hormones are specific hormones that stimulate the release of other hormones from the pituitary gland. B: Incorrect. Adenohypophysis secretes different hormones, not these specific ones. C: Incorrect. Insulin, for example, decreases blood glucose levels, while cortisol increases blood glucose levels.

Correct Answer: B

Rationale: The correct answer is B: stimulates osteoclastic activity. Parathyroid hormone (PTH) acts on bones to stimulate osteoclasts, which are responsible for breaking down bone tissue to release calcium into the bloodstream. This process helps increase blood calcium levels. Choices A, C, and D are incorrect because PTH actually increases plasma calcium levels, is not an iodine-containing hormone, and does not stimulate osteoblastic activity.