**Core Concept**
Pancytopenia refers to reduced counts of all three blood cell types (red blood cells, white blood cells, and platelets). While bone marrow hypoplasia or dysfunction can cause pancytopenia, the presence of **cellular marrow** (active bone marrow production) rules out conditions with marrow failure. This question tests understanding of which disorders cause pancytopenia despite active marrow involvement.

**Why the Correct Answer is Right**
G6PD deficiency primarily causes **hemolytic anemia** due to oxidative stress-induced red blood cell destruction, not pancytopenia. It does not involve marrow failure or dysplasia. The bone marrow remains **hypercellular** in response to chronic hemolysis, producing more RBCs. Thus, despite anemia, there is no pancytopenia. In contrast, megaloblastic anemia (B), myelodysplasia (B), and paroxysmal nocturnal hemoglobinuria (C) all show pancytopenia with cellular marrow due to ineffective erythropoiesis or marrow dysplasia.

**Why Each Wrong Option is Incorrect**
Option A: Megaloblastic anemia causes pancytopenia due to impaired DNA synthesis (vitamin B12/folate deficiency), leading to dysplastic cells and ineffective hematopoiesis, with cellular marrow.
Option B: Myelodysplasia features dysplastic cells and pancytopenia with active marrow involvement.
Option C: Paroxysmal nocturnal hemoglobinuria causes intravascular hemolysis and pancytopenia with cellular marrow due to ineffective erythropoiesis and marrow infiltration.

**Clinical Pearl / High-Yield Fact**
G6PD deficiency causes **hemolytic anemia**, not pancytopenia — always remember it presents with **acute hemolysis**, not low WBCs or platelets. Pancytopenia with cellular marrow is a hallmark of **dysplastic or hemolytic disorders**, not G6PD deficiency.

✓ Correct Answer: D. G6PD deficiency