Which of the following drugs is used in conjunction with diet therapy to reduce a patient's serum cholesterol level?

Azithromycin is an antibiotic used to treat various bacterial infections. It does not have any effect on cholesterol levels or lipid metabolism, making it irrelevant for managing hypercholesterolemia or cholesterol-related conditions.

Lisinopril is an ACE inhibitor primarily used to treat high blood pressure and heart failure. While it may have some cardiovascular protective effects, it does not directly lower serum cholesterol levels and is not indicated for cholesterol management.

Sertraline is a selective serotonin reuptake inhibitor (SSRI) used to treat depression and anxiety disorders. It does not have any pharmacological action related to cholesterol reduction and is not utilized for managing lipid levels.

Zolpidem is a sedative-hypnotic medication used to treat insomnia. It has no impact on cholesterol levels and is not relevant for any treatment aimed at lowering serum cholesterol, making it unsuitable for this context.