A 24-month-old girl from an urban slum in Delhi presents with a 4-month history of poor weight gain, loose stools, and recurrent respiratory infections. On examination, she weighs 8 kg (expected 12.5 kg), height 80 cm (expected 87 cm), and mid-arm circumference 12 cm. There is no edema, but the child appears wasted. The mother asks what investigations should be done to confirm malnutrition and assess for complications. Which single investigation is most appropriate to detect the most common complication of chronic protein-energy malnutrition in this age group?