This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
All the following are preliminary investigations in primary coagulation defects except - 
Select and explain. 
A. Bleeding time, B. Platelet count, C. Prothrombin time, D. Platelet aggregation studies
C: Prothrombin time
Answer: C: Prothrombin time 
Explanation: None