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

**Why the Correct Answer is Right:** ADH acts on the collecting ducts in the nephrons of the kidneys, specifically the magnocellular neurons of the neurohypophysis. It binds to the V2 receptors, leading to secondary activation of adenylate cyclase, which in turn increases intracellular cyclic AMP levels. This activates the chloride channel (ASIC2a) and the aquaporin-2 channel, resulting in water reabsorption and reduced urine output.

**Why Each Wrong Option is Incorrect:**

A. Kidney tubules: Although ADH does affect the kidneys, it specifically acts on the collecting ducts, not the nephrons.
B. Neurons: ADH does act on neurons, but only on the magnocellular neurons of the neurohypophysis, not on all neurons.
C. Osmoreceptors: ADH is released in response to increased osmolality, not directly acting on osmoreceptors.
D. Whole body: ADH primarily regulates the kidneys, not the entire body.

**Clinical Pearl:** ADH's specific action on the collecting ducts helps maintain the body's water and electrolyte balance, ensuring proper fluid homeostasis and preventing dehydration. Understanding ADH's role in the context of osmoregulation is crucial for clinical practice, particularly in assessing and treating patients with disorders affecting fluid and electrolyte balance.