Pancytopenia is most common after:

Hepatitis is the most common preceding infection, and post hepatitis marrow failure accounts for about 5% of etiologies in most series. Patients are usually young men who have recovered from a bout of liver inflammation 1 to 2 months earlier; the subsequent pancytopenia is very severe. The hepatitis is seronegative (non-A, non-B, non-C, non-G) and possibly due to a novel, as yet undiscovered, virus. Ref: Harrison's principle of internal medicine 17th edition, chapter 102.