Which of the following is the commonest cause of salt wasting nephropathy?
**Core Concept**
Salt wasting nephropathy is a rare kidney disorder characterized by excessive loss of sodium, chloride, and water in the urine, leading to dehydration and electrolyte imbalances. This condition is often associated with congenital anomalies or acquired injuries to the kidneys.
**Why the Correct Answer is Right**
The correct answer is Congenital Adrenal Hyperplasia (CAH) due to 21-hydroxylase deficiency. In CAH, the adrenal glands produce excessive amounts of cortisol and aldosterone precursors, which are then converted to cortisol and aldosterone in the liver. However, the excess cortisol production inhibits the release of adrenocorticotropic hormone (ACTH) from the pituitary gland, leading to decreased production of mineralocorticoids, including aldosterone. As a result, the kidneys fail to reabsorb sodium and water, causing salt wasting nephropathy.
**Why Each Wrong Option is Incorrect**
* **Option A:** Autosomal Dominant Polycystic Kidney Disease (ADPKD) is a genetic disorder characterized by the growth of numerous cysts in the kidneys, but it is not typically associated with salt wasting nephropathy.
* **Option B:** Diabetes Insipidus is a condition characterized by excessive thirst and urination due to a deficiency of antidiuretic hormone (ADH), but it does not typically cause salt wasting nephropathy.
* **Option C:** Medullary Sponge Kidney is a congenital disorder of the kidneys characterized by cystic dilatation of the collecting ducts, but it is not typically associated with salt wasting nephropathy.
**Clinical Pearl / High-Yield Fact**
Salt wasting nephropathy is a rare but potentially life-threatening condition that requires prompt recognition and treatment. Remember that CAH due to 21-hydroxylase deficiency is the most common cause of salt wasting nephropathy, and it often presents with symptoms such as dehydration, electrolyte imbalances, and failure to thrive in infants.
**Correct Answer:** C.