What is the mechanism of action for hydrochlorothiazide?

Hydrochlorothiazide is a thiazide diuretic that primarily acts on the kidneys to promote the excretion of sodium and chloride. The drug inhibits the sodium-chloride symporter in the distal convoluted tubule of the nephron, which prevents the reabsorption of these ions back into the bloodstream. As a result, sodium and chloride remain in the renal tubules and are excreted in urine, leading to an increase in urine output. This action not only helps in managing conditions like hypertension and edema but also contributes to electrolyte balance by influencing fluid and sodium levels in the body.

The other provided options do not accurately describe the drug's mechanism of action. For example, increasing sodium reabsorption opposes the effect of hydrochlorothiazide, which is to increase sodium excretion. Inhibiting potassium reabsorption also does not describe the primary action of hydrochlorothiazide; rather, thiazide diuretics may lead to potassium loss in some patients. Reducing blood volume is a secondary effect of increased sodium excretion and is not the direct mechanism of action of hydrochlorothiazide itself. Thus, B represents the accurate and primary mechanism by which hydrochlorothiazide operates.