Two 3-year-old children are evaluated for growth faltering. Child A has a weight-for-age of 60% of expected, height-for-age of 95% of expected, and normal mid-arm circumference. Child B has a weight-for-age of 75% of expected, height-for-age of 75% of expected, and reduced mid-arm circumference. Which anthropometric finding best discriminates between constitutional growth delay in Child A and nutritional stunting in Child B?