Which of the following medications is classified as a cholesterol-lowering agent?

Atorvastatin is classified as a cholesterol-lowering agent, specifically belonging to a group of medications known as statins. Statins work by inhibiting the enzyme HMG-CoA reductase, which plays a central role in cholesterol synthesis in the liver. By blocking this enzyme, atorvastatin effectively reduces the levels of LDL cholesterol (often referred to as "bad" cholesterol) and triglycerides in the blood, while also modestly increasing HDL cholesterol (the "good" cholesterol). This mechanism helps in lowering the risk of cardiovascular diseases, including heart attacks and strokes.

The other medications listed do not fall into the category of cholesterol-lowering agents. Cotrimoxazole is an antibiotic used to treat various bacterial infections, metoprolol is a beta-blocker often prescribed for hypertension and heart conditions, and hydrochlorothiazide is a thiazide diuretic used primarily to manage high blood pressure and reduce fluid retention. Thus, atorvastatin is distinctly recognized for its specific action in cholesterol reduction, making it the correct choice among the options provided.