**Core Concept**
The patient's presentation of microcytic hypochromic anemia with a history of frequent blood transfusions suggests iron deficiency anemia, but the possibility of hemoglobinopathies or other causes like chronic disease, lead poisoning, or sideroblastic anemia cannot be ruled out. The patient's laboratory values, including low hemoglobin, low mean corpuscular volume (MCV), and low red blood cell (RBC) count, further support this diagnosis.

**Why the Correct Answer is Right**
Given the patient's presentation, the most likely diagnosis is iron deficiency anemia. However, the patient's history of frequent blood transfusions may have masked the underlying cause of the anemia. In this case, a serum ferritin level would be the most useful investigation to confirm iron deficiency anemia, as it directly measures the body's iron stores. The other investigations listed (e.g., serum iron, total iron-binding capacity (TIBC), and lead levels) may provide additional information but are not as critical in this scenario.

**Why Each Wrong Option is Incorrect**
**Option A:** Serum iron levels may be low in iron deficiency anemia, but they can also be low in other conditions such as anemia of chronic disease. Therefore, serum iron levels alone may not be sufficient to confirm the diagnosis of iron deficiency anemia.
**Option B:** Total iron-binding capacity (TIBC) is a useful investigation in diagnosing iron deficiency anemia, as it is elevated in cases of iron deficiency. However, in patients with a history of frequent blood transfusions, TIBC may be low due to the transfused iron.
**Option C:** Lead levels are an important investigation in cases of microcytic hypochromic anemia, especially in children. However, in adults with a history of frequent blood transfusions, lead poisoning is less likely and therefore not as critical an investigation.

**Clinical Pearl / High-Yield Fact**
In patients with a history of frequent blood transfusions, it is essential to consider the possibility of iron overload or hemoglobinopathies, which can masquerade as iron deficiency anemia.

**Correct Answer: D. Lead levels.**