Final answer:
Before prescribing spironolactone, you must check serum potassium and creatinine levels.
Step-by-step explanation:
Before prescribing spironolactone, you must check two things: serum potassium and creatinine. Spironolactone is a medication that belongs to a class of drugs called aldosterone antagonists, which work by blocking the action of aldosterone, a hormone that regulates various electrolytes in the body. Checking serum potassium is important because spironolactone can cause an increase in potassium levels, which can be dangerous. Additionally, checking creatinine is necessary to assess kidney function, as spironolactone is mainly excreted by the kidneys.