Most common cause of aplastic anemia is

Aplastic anemia is pancytopenia with bone marrow hypocellularity. Acquired aplastic anemia is distinguished from iatrogenic aplasia, marrow hypocellularity after intensive cytotoxic chemotherapy for cancer, and from usually accidental physical and chemical injury, as in radiation poisoning. Most cases of aplastic anemia are idiopathic, little other than history separates these cases from those with a presumed etiology such as a drug exposure. ref - Harrisons internal medicine 20e pg 724