A 2-year-old girl from rural Maharashtra is brought to the primary health center with a history of poor weight gain since 6 months of age. Her weight is 10 kg (below 5th percentile) and height is 80 cm (10th percentile). The mother reports inadequate dietary intake and recurrent infections. On anthropometric examination, mid-upper arm circumference (MUAC) is 12.5 cm. Which is the most appropriate investigation to assess the severity and type of protein-energy malnutrition (PEM)?