What ion is primarily affected by the action of hydrochlorothiazide?

Hydrochlorothiazide is a thiazide diuretic primarily affecting sodium reabsorption in the distal convoluted tubule of the nephron in the kidneys. Its mechanism of action involves inhibiting the sodium-chloride symporter, leading to increased excretion of sodium and chloride in the urine. As a result, this diuretic effectively lowers the reabsorption of sodium, increasing its concentration in the urine.

The impact on sodium levels is crucial, as it contributes to its diuretic effect, helping to manage conditions such as hypertension and edema. While hydrochlorothiazide also influences potassium and calcium levels, its primary and most significant action is associated with sodium. Therefore, sodium is the correct answer as it reflects the main pharmacological target of hydrochlorothiazide therapy.