<prompt> A 10 year old boy presents with mucosal bleeding of 1 week duration. The investigation of choice that will be most useful in him isProthrombin timeClotting timePartial thromboplastin timePlatelet count  <response>: Bleeding from mucosal surface suggests thrombocytopenia or defect in platelet function.First do platelet count, if there is thrombocytopenia, further investigation will be required. 