What is the effect of an increase in ADH on urine output?

An increase in Antidiuretic Hormone (ADH), also known as vasopressin, leads to a decrease in urine output. ADH plays a crucial role in the body's water balance by increasing the permeability of the renal collecting ducts to water. When ADH levels rise, more water is reabsorbed back into the circulation, leading to more concentrated urine and reduced urine volume. This mechanism allows the body to conserve water, which is particularly important in response to dehydration or increased plasma osmolality. Therefore, the correct answer accurately reflects this physiological response to elevated ADH levels.