A 18-month-old child from rural Bihar presents with severe wasting, loss of subcutaneous fat, and skin changes. The child weighs 7 kg (expected 12 kg for age). On examination, there is generalized edema, hepatomegaly, and sparse hair. The mother reports the child has been fed dilute rice water for the past 6 months due to recurrent diarrhea. Which investigation is most specific for confirming the diagnosis of protein-energy malnutrition and assessing severity?