A 2-year-old girl from a slum in Delhi presents with a 9-month history of progressive weight loss, chronic diarrhoea, and recurrent respiratory infections. On examination, weight is 8 kg (expected 12.5 kg), height 78 cm (expected 87 cm), with severe muscle wasting, loose skin folds, and absence of subcutaneous fat. Serum albumin is 3.2 g/dL, total protein 5.8 g/dL, and blood glucose 55 mg/dL. There is no oedema. What is the most likely diagnosis?