**Core Concept**
The patient's presentation of repeated episodes of sore throat, anemia (Hb 9.0 g/dL), and pancytopenia (low WBC and platelet count) with a normal MCV suggests a chronic bone marrow failure syndrome, likely aplastic anemia or myelodysplastic syndrome (MDS).

**Why the Correct Answer is Right**
The best approach to diagnosis involves a bone marrow biopsy to assess the cellular morphology, cytogenetics, and molecular genetics of the bone marrow cells. This will help differentiate between aplastic anemia, MDS, and other conditions such as paroxysmal nocturnal hemoglobinuria (PNH). The bone marrow examination will also guide further management, including the possibility of hematopoietic stem cell transplantation.

**Why Each Wrong Option is Incorrect**
**Option A:** Peripheral blood smear examination may provide some clues about the underlying condition, but it is not sufficient for a definitive diagnosis of aplastic anemia or MDS.
**Option B:** Flow cytometry can be useful in diagnosing PNH, but it is not the best initial approach for diagnosing aplastic anemia or MDS.
**Option C:** Complete blood count (CBC) with differential is essential for assessing pancytopenia, but it does not provide information about the underlying cause of bone marrow failure.

**Clinical Pearl / High-Yield Fact**
Aplastic anemia is a rare but life-threatening condition that requires prompt diagnosis and treatment. The diagnosis of aplastic anemia is often delayed due to its nonspecific presentation, but a high index of suspicion and prompt bone marrow examination can lead to timely treatment and improved outcomes.

**Correct Answer:** C. Complete a bone marrow biopsy to assess cellular morphology, cytogenetics, and molecular genetics.