The parent brings a child to the clinic for a 12-month well visit. The child weighed 6 lb 2 ounces and was 21 inches long at birth. Which finding will cause the nurse to intervene?

A. Growth in length (30 inches) is appropriate—average height gain is about 50% from birth (21 in → ~31.5 in at 12 months). B. By 12 months, an infant's weight should triple their birth weight. This child should weigh about 18–19 lb, but only weighs 16 lb, indicating failure to thrive or nutritional concern. C. Potty training is not developmentally expected at 12 months; it typically begins around 18–24 months. D. Walking up stairs is expected around 18–24 months. The lack of this ability is age-appropriate at 12 months.