A 18-month-old boy from an urban slum in Delhi is admitted with a 4-month history of progressive weight loss, chronic diarrhea, and recurrent respiratory tract infections. On examination, he weighs 7.5 kg (expected 11–12 kg), with severe loss of subcutaneous fat and muscle wasting. His ribs are prominently visible, and his skin hangs loosely over his body. There is no edema. Serum albumin is 2.8 g/dL, and total serum protein is 5.2 g/dL. His mother reports inadequate feeding due to poverty and poor sanitation. What is the most likely diagnosis?