**Question:** In PCT (Posterior Medulla), sodium reabsorption is prevented by:

**Core Concept:** The Nephron is the functional unit of the kidney, responsible for filtering blood, maintaining electrolyte balance, and regulating fluid volume. The PCT (Posterior Medulla) is a part of the renal medulla where essential mechanisms of sodium reabsorption occur.

**Why the Correct Answer is Right:** In PCT, sodium reabsorption is prevented by the action of antidiuretic hormone (ADH) or vasopressin. ADH is released by the hypothalamus in response to low blood osmolality or high blood pressure, and acts on the collecting duct.

**Why Each Wrong Option is Incorrect:**

A. **Angiotensin II:** While angiotensin II is released by the renin-angiotensin-aldosterone system (RAAS) in response to high blood pressure, it does not directly prevent sodium reabsorption in PCT.

B. **Aldosterone:** Aldosterone is involved in sodium reabsorption in the distal convoluted tubule (DCT) and collecting duct (CD), not in the PCT.

C. **Atrial Natriuretic Peptide (ANP):** ANP is released in response to high blood pressure, but it does not specifically target the PCT for sodium reabsorption inhibition.

D. **Diuretic hormone:** This term does not accurately describe the hormone responsible for sodium reabsorption inhibition in PCT.

**Clinical Pearl:** Inhibition of sodium reabsorption in PCT ensures the conservation of sodium, which is essential for maintaining blood pressure, fluid volume, and osmolality balance. By preventing sodium reabsorption in the PCT, ADH helps maintain the osmotic gradient necessary for urine formation, ultimately leading to urine concentration and conservation of water.

**Correct Answer:** **D. Vasopressin (ADH)**

Vasopressin, also known as antidiuretic hormone (ADH), plays a critical role in regulating fluid balance and maintaining blood pressure by preventing sodium reabsorption in the PCT. It is released in response to low blood osmolality or high blood pressure, ensuring that water is reabsorbed at the CD and not in the PCT. This helps maintain the osmotic gradient necessary for urine concentration and water conservation.