Pancytopenia with cellular marrow is seen in all, except:
G6PD deficiency is not associated with pancytopenia and a cellular marrow. So G6PD is the single best answer of choice. Ref: Harrison's Principles of Internal Medicine, 16th Edition, Page 617; Manual of Practical Medicine By R Alagappan, 3rd Edition, Page 335
Megaloblastic anemia
Myelodysplasia
Paroxysmal nocturnal hemoglobinuria
G6 PD deficiency