Correct Answer: D

Rationale: The renin-angiotensin-aldosterone axis does not involve the hypothalamus or pituitary gland; instead, it primarily involves the kidneys and the renin-secreting cells. In this axis, renin is released in response to low blood pressure, leading to the formation of angiotensin II and the subsequent release of aldosterone from the adrenal glands. Feedback mechanisms for this axis primarily involve the regulation of renin release based on blood pressure rather than involving direct negative feedback from downstream hormones on the hypothalamus or pituitary gland. On the other hand, choices A, B, and C all involve classic examples of hypothalamic-pituitary negative feedback loops where the end hormone suppresses the release of the stimulating hormones at the hypothalamic and pituitary levels.

Correct Answer: A

Rationale: While African race is considered a protective factor against the development of osteoporosis due to higher peak bone mass and lower fracture rates compared to other races, the other factors listed (current cigarette smoking, female sex, and physical inactivity) are all established risk factors for the development of osteoporotic fractures. African-American individuals, in general, have a lower risk of osteoporosis and fractures compared to Caucasians and Asians.

Correct Answer: C

Rationale: Cushing's syndrome is a collection of signs and symptoms caused by prolonged exposure to high levels of cortisol. Common features of Cushing's syndrome include weight gain, central obesity, moon face, buffalo hump, easy bruising, muscle weakness, and menstrual irregularities in women.

Correct Answer: B

Rationale: Postmenopausal estrogen therapy has been shown to have a protective effect on bone density and reduce the risk of osteoporosis-related fractures, including hip fractures. Estrogen therapy helps to maintain bone strength and reduce the chances of fractures in postmenopausal women. Therefore, estrogen therapy does not increase the risk of hip fractures; in fact, it may decrease the risk of hip fractures in this population.

Correct Answer: D

Rationale: Blood calcium levels are primarily regulated by the actions of parathyroid hormone (PTH) and Vitamin D. Vitamin D plays a crucial role in maintaining calcium balance in the body by promoting the absorption of calcium from the intestines. When blood calcium levels are low, Vitamin D works to increase absorption of calcium to raise the levels back to normal. Therefore, elevated blood calcium levels are commonly associated with excess Vitamin D intake or conditions that increase Vitamin D levels, such as hypervitaminosis D.