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
A 22 year old man presents with history of bleeding from gums for the last 6 months. On investigation the b was found to be 8.2 gm% TLC 4400/mm, DLC N 64%, L 27%, E 3%, M 6% and platelet count of 20,000 /cu mm. Which one of the following investigation would be most useful in establishing the diagnosis - 
Select and explain. 
A. Bleeding time and clotting time, B. Prothrombin time, C. Partial thromboplastin time, D. Bone marrow examination
D: Bone marrow examination
Answer: D: Bone marrow examination 
Explanation: None