SIADH is caused by all except –
**Core Concept**
The Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) is a condition characterized by excessive release of antidiuretic hormone (ADH), also known as vasopressin, leading to water retention and hyponatremia. ADH stimulates the kidneys to reabsorb water, resulting in a dilutional hyponatremia.
**Why the Correct Answer is Right**
SIADH can be caused by various conditions that stimulate the release of ADH or mimic its effects. These conditions include ectopic production of ADH by tumors, such as small cell lung cancer, and central nervous system disorders, such as traumatic brain injury or subarachnoid hemorrhage. The hypothalamus and posterior pituitary gland are responsible for regulating ADH release, and any disruption in this pathway can lead to SIADH.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a common cause of SIADH. While certain medications can cause SIADH, this option is not typically listed as a cause.
**Option B:** This option is incorrect because it is actually a common cause of SIADH. Ectopic ADH production by tumors is a well-known cause of the syndrome.
**Option C:** This option is incorrect because it is a common cause of SIADH. Central nervous system disorders, such as traumatic brain injury, can stimulate ADH release and lead to SIADH.
**Clinical Pearl / High-Yield Fact**
One key aspect of SIADH is that the serum osmolality is usually in the normal range, despite the presence of hyponatremia. This is because the kidneys are able to concentrate the urine in response to ADH, maintaining a normal serum osmolality.
**Correct Answer:** A.