A 3-year-old boy from rural Maharashtra is brought to the primary health centre with a 6-month history of poor weight gain, recurrent infections, and diarrhoea. On examination, he weighs 10 kg (expected 14 kg for age), height 85 cm, appears alert and interactive, with mild bilateral pitting oedema over the shins. Serum albumin is 2.8 g/dL (normal 3.5–5.0), total protein 5.2 g/dL, and haemoglobin 9.2 g/dL. What is the most likely diagnosis?