Chronic diarrhea in infancy; differential diagnosis

A 18-month-old girl is brought to the pediatric clinic with a 6-week history of persistent diarrhea. She has had 4–5 loose stools daily, with no blood or mucus. On examination, she appears mildly dehydrated with a weight loss of 10% from her baseline. Her mother reports that the child was exclusively breastfed until 6 months of age and was introduced to cow's milk at 12 months. Laboratory investigations reveal: hemoglobin 9.2 g/dL, serum albumin 2.8 g/dL, and fecal chymotrypsin is normal. Stool culture and ova-cyst examination are negative. Which of the following is the most likely diagnosis?