A nine-month-old boy of Sindhi parents presented to you with complaints of progressive lethargy, irritability and pallor since 6 months of age. Examination revealed severe pallor. Investigation showed Hb-3.8 g%; MCV- 58 fL, MCH-19.4 pg/cell. Osmotic fragility is normal; Blood film shows target cells and normoblasts. X-ray skull shows expansion of erythroid marrow. Which of the following is the most likely diagnosis:

Discussing options one by one: a. Iron deficiency anemia: So severe anemia & expansion of erythroid marrow are usually not seen b. Acute lymphoid Leukemia: Low MCV, no obvious involvement of other cell lines, and expansion of erythroid marrow are incompatible with diagnosis of ALL c. Hemoglobin D disease: Low MCV and MCH with target cells suggest abnormality of haemoglobin like HbD disease d. Hereditary spherocytosis: Normal osmotic fragility goes against hereditary spherocytosis