Okay, let's tackle this question about the site of action of vasopressin. First, I need to recall what vasopressin does. Vasopressin, also known as antidiuretic hormone (ADH), is a hormone released by the posterior pituitary. Its main role is in water reabsorption in the kidneys.

So, where exactly does it act? I remember that ADH acts on the collecting ducts of the nephron. Specifically, it increases the permeability of the collecting ducts to water by inserting aquaporin-2 channels into the apical membrane of the principal cells. This allows water to be reabsorbed back into the bloodstream, concentrating the urine and conserving water.

Now, looking at the options, the correct answer should be the collecting duct. The other options might include other parts of the nephron like the proximal convoluted tubule, loop of Henle, or distal convoluted tubule. Let's verify each. The proximal convoluted tubule reabsorbs about 65% of filtered water, but this is passive and not under ADH's control. The loop of Henle is involved in creating the medullary osmotic gradient, mainly through the countercurrent multiplier system. The distal convoluted tubule is part of the diluting segment and isn't responsive to ADH. The collecting duct, however, is where ADH exerts its effect, so that's the right answer.

Clinical pearl: Remember that diabetes insipidus, which is a deficiency of ADH or its receptors, leads to inability to concentrate urine, resulting in large volumes of dilute urine. This directly relates to the collecting duct's role under ADH influence.

**Core Concept**
Vasopressin (antidiuretic hormone, ADH) regulates water reabsorption in the kidneys. Its primary site of action is the **collecting ducts** of the nephron, where it increases aquaporin-2 insertion in principal cells to enhance water reabsorption.

**Why the Correct Answer is Right**
Vasopressin binds to **V2 receptors** on principal cells in the collecting ducts. This activates adenylate cyclase, increasing cAMP and triggering aquaporin-2 (AQP2) channels to migrate to the apical membrane. Water then moves osmotically from the tubular lumen into the interstitial space, conserving body water. The collecting duct is uniquely responsive to ADH, allowing urine concentration.

**Why Each Wrong Option is Incorrect**
**Option A:** Proximal convoluted tubule – Water reabsorption here is passive and not regulated by ADH.
**Option B:** Loop of Henle – This segment establishes the medullary osmotic gradient via countercurrent multiplication, not ADH.
**Option C:** Distal convoluted tubule – This segment is impermeable to water in the absence of ADH and lacks aquaporins.

**Clinical Pearl / High-Yield Fact**
Diabetes insipidus (DI) results from ADH deficiency (central DI) or resistance (nephrogenic DI), leading to polyuria and dilute urine. The collecting duct’s role in ADH-dependent water reabsorption is critical for diagnosis and management.

✓ Correct Answer: C. Distal tubule