A nurse is reviewing the laboratory values of a client who is taking atorvastatin. Which of the following laboratory values indicates the treatment has been effective?

Atorvastatin is an HMG-CoA reductase inhibitor that lowers cholesterol by blocking the rate-limiting step of hepatic cholesterol synthesis. Its primary therapeutic goal is the reduction of atherogenic lipoproteins to prevent cardiovascular disease and stroke. Effective therapy is measured by a significant improvement in the lipid profile, specifically targeting the reduction of low-density lipoproteins (LDL). Rationale: A. Decreased HDL levels would indicate a worsening of the patient's cardiovascular risk profile rather than effective treatment. High-density lipoprotein (HDL) is considered "good" cholesterol because it helps remove other forms of cholesterol from the bloodstream. A successful response to statin therapy would involve maintaining or slightly increasing HDL levels to provide better cardioprotective benefits. B. Increased serum transaminase levels are an adverse effect of atorvastatin indicating potential hepatotoxicity, not therapeutic effectiveness. Statins are metabolized by the liver and can occasionally cause inflammation or damage to hepatocytes. If these enzymes (AST and ALT) rise significantly, the nurse must report this finding as it may necessitate the discontinuation of the medication. C. Decreased LDL levels are the hallmark indicator of effective atorvastatin therapy. Low-density lipoprotein is the primary target for statins because it contributes to plaque buildup in the arteries, leading to atherosclerosis. A significant reduction in LDL proves that the HMG-CoA reductase inhibitor is successfully lowering the production of harmful cholesterol and reducing overall cardiovascular risk. D. Increased creatine kinase (CK) levels are a laboratory marker for muscle damage and a sign of a serious adverse effect called rhabdomyolysis. This condition involves the breakdown of skeletal muscle fibers, which can lead to acute renal failure. Elevated CK levels are a reason for concern and immediate clinical intervention, not a sign that the cholesterol-lowering treatment is working.