A 3-year-old child from a rural area in Uttar Pradesh presents with severe acute malnutrition (SAM), with a mid-upper arm circumference (MUAC) of 10.5 cm and clinical signs of edema. The child is being enrolled in the Integrated Management of Neonatal and Childhood Illness (IMNCI) program. Which is the most appropriate investigation to assess the severity and guide nutritional rehabilitation?