A 18-month-old girl from urban Delhi is evaluated for poor weight gain. Birth weight was 3.2 kg (normal). At 6 months, her weight was 6.8 kg (50th percentile). Current weight is 8.5 kg (25th percentile for age), height is 78 cm (50th percentile for age), and MUAC is 13.5 cm. Mother reports that the child had recurrent episodes of diarrhea over the past 6 months and was started on cow's milk (diluted with water) 3 months ago after weaning from breast milk. The child appears wasted with visible ribs and minimal subcutaneous fat. What is the most likely cause of this child's current nutritional status?