What effect does furosemide primarily have on the kidneys?

Furosemide is a powerful loop diuretic that primarily acts on the kidneys by inhibiting the reabsorption of sodium and chloride in the ascending loop of Henle. This action leads to increased excretion of sodium and water, resulting in a significant diuretic effect. By preventing sodium reabsorption, furosemide effectively promotes diuresis, which is the increased production of urine.

In this context, the first option is correct because the mechanism of action of furosemide directly relates to its ability to decrease the reabsorption of sodium and water, thereby increasing their excretion in the urine. This characteristic is what makes furosemide particularly useful in conditions such as heart failure, where fluid overload is a concern.

Other choices do not accurately represent the primary effect of furosemide. For example, increasing potassium retention is not an effect of furosemide; instead, it often leads to increased potassium excretion. Similarly, decreasing urine output contradicts the primary action of a diuretic like furosemide, which is to promote urine production, and increasing calcium absorption is not relevant to the mechanism by which furosemide operates.