**Question:** In the presence of ADH, greatest fraction of the filtered water is reabsorbed from:

A. Distal convoluted tubule
B. Proximal convoluted tubule
C. Loop of Henle
D. Collecting duct

**Core Concept:** Antidiuretic Hormone (ADH), also known as vasopressin, is a hormone secreted by the posterior pituitary gland in response to increased osmolality or decreased blood volume. Its primary function is to regulate water reabsorption in the nephrons of the kidneys.

**Why the Correct Answer is Right:** In the nephron, ADH acts primarily on the collecting ducts. It increases the permeability of the collecting ducts to water, allowing more water to be reabsorbed into the bloodstream. Among the given options, D (Collecting duct) is the correct answer because it is the site where ADH exerts its major effect on water reabsorption.

**Why Each Wrong Option is Incorrect:**

A. Distal convoluted tubule: Although ADH does affect the distal convoluted tubule to some extent, the primary action of ADH is on the collecting ducts, not the distal convoluted tubule.

B. Proximal convoluted tubule: The proximal convoluted tubule is responsible for reabsorbing electrolytes like sodium, potassium, and glucose. It does not play a significant role in water reabsorption.

C. Loop of Henle: The loop of Henle is an anatomical structure, not a site of ADH action. It is part of the nephron, which helps in concentrating urine by creating a concentration gradient.

**Clinical Pearl:** ADH plays a critical role in maintaining the body's fluid and electrolyte balance by increasing the permeability of the collecting ducts to water. This helps in concentrating urine and prevents dehydration in conditions like diabetes insipidus, where ADH production is deficient or impaired.

**Correct Answer:** D. Collecting duct

**Explanation:** In the nephron, ADH targets the collecting ducts to increase their permeability for water, allowing more water to be reabsorbed into the bloodstream. This action contributes to concentrating urine and maintaining fluid and electrolyte balance in the body.