A client who was recently diagnosed with atrial fibrillation is receiving warfarin. To see the drug's effectiveness, which laboratory finding should the practical (PN) review?

A. Complete blood count: Primarily measures the number of blood cells and their types. It does not directly assess the effectiveness of warfarin. B. Creatinine clearance: Evaluates kidney function and doesn't directly reflect the efficacy of warfarin. C. Prothrombin time (PT): Measures the time taken for blood to clot, an essential indicator for monitoring anticoagulation therapy like warfarin. D. Serum troponin levels: Assesses cardiac damage and is not indicative of warfarin's effectiveness in managing atrial fibrillation.