Commonest cause of phaeochromocytoma is:

Correct Answer: A

Rationale: Phaeochromocytoma is a neuroendocrine tumor that arises from the chromaffin cells of the adrenal medulla. It is the most common cause of a catecholamine-secreting tumor in adults. While it can also rarely occur outside the adrenal gland (referred to as extra-adrenal or paraganglioma), the majority of cases originate within the adrenal medulla. Symptoms of phaeochromocytoma are due to the excessive release of catecholamines (epinephrine and norepinephrine), leading to a characteristic triad of headaches, sweating, and palpitations. Therefore, the commonest cause of phaeochromocytoma is a tumor of the adrenal medulla.

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: C

Rationale: Oxytocin is a hormone that is produced in both males and females. In females, oxytocin plays a crucial role in various physiological processes, notably during labor and childbirth. Oxytocin acts on the smooth muscles of the uterus, causing them to contract during labor (parturition) and helps in the ejection of milk during breastfeeding. This hormone is primarily synthesized in the paraventricular nucleus (PVN) and the supraoptic nucleus (SON) of the hypothalamus and stored in the posterior pituitary gland before being released into the bloodstream. Overall, oxytocin's main function is to stimulate smooth muscle contraction in the uterus and the mammary glands.

Correct Answer: A

Rationale: Phaeochromocytoma is a neuroendocrine tumor that arises from the chromaffin cells of the adrenal medulla. It is the most common cause of a catecholamine-secreting tumor in adults. While it can also rarely occur outside the adrenal gland (referred to as extra-adrenal or paraganglioma), the majority of cases originate within the adrenal medulla. Symptoms of phaeochromocytoma are due to the excessive release of catecholamines (epinephrine and norepinephrine), leading to a characteristic triad of headaches, sweating, and palpitations. Therefore, the commonest cause of phaeochromocytoma is a tumor of the adrenal medulla.

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.