参考答案：C

解析：Aminoglycosides such as gentamicin have a narrow range between therapeutic and toxic serum levels. A serum peak and trough level (taken half an hour before the dose and half an hour after the dose has been administered) around the third dose (the third dose provides enough medication build up in the blood stream to be measured) is the most accurate way to determine the correct serum values. A trough level every morning, a serum peak level after the second dose, and serial serum trough levels won't provide sufficient data about the effectiveness of the antibiotic.