A 18-month-old girl from an urban slum in Delhi presents with a weight-for-age of 60% of the expected median (by WHO growth standards) and a height-for-age of 75% of expected. Her mother reports that the child has been exclusively breastfed until 12 months and now receives irregular complementary feeding. On examination, the child is alert but has visible wasting and mild edema of the feet. Hemoglobin is 8.2 g/dL. What is the drug of choice for addressing the most critical micronutrient deficiency in this child?