A 12-month-old girl who has trisomy 18 presents with 3 weeks of intermittent fever, emesis, and irritability, but no diarrhea, rash, lethargy, or seizure activity. In addition, she has lost nearly 300 g over the previous 3 months. Her medical history includes repair of tetralogy of Fallot, first-degree atrioventricular block, chronic lung disease, seizures, and gastroesophageal reflux. Medications include phenytoin and inhaled corticosteroids.

Physical examination reveals a small, irritable, dysmorphic child who has a temperature of 101.1°F (38.4°C) and blood pressure of 122/57 mm Hg. Height, weight, and head circumference are below the third percentile. There is a 3/6 systolic regurgitant murmur. Abdominal examination is limited due to pacemaker placement in the right upper quadrant, but the liver is palpated 3 cm below the costal margin and spleen at 2 cm. The remainder of the physical findings are normal.

CBC is normal except for a WBC count of 18.5×103...

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