Review Question - QID 109173

A 14-day-old girl is brought to the emergency department for diarrhea. Her mother reports that the patient began having “explosive” diarrhea 6 hours ago. The patient has little interest in feeding since leaving the hospital after her birth and has not yet regained her birth weight. Her immediate postnatal period was otherwise significant for 2 episodes of bilious vomiting that resolved with the temporary use of a nasogastric tube and expulsion of stool during a digital rectal exam. The patient passed meconium on the 3rd day of life. Her temperature is 101.8°F (38.8°C), blood pressure is 58/37 mmHg, pulse is 148/min, and respirations are 38/min. On physical exam, the patient appears to be in moderate distress. She has no dysmorphic features and she has a normal S1/S2 without any murmurs, rubs, or gallops. Her abdomen is firm and distended. A digital rectal exam reveals a tight anal sphincter and precipitates a release of gas and liquid stool. Her abdominal radiograph can be seen in Figure A. Which of the following is the most accurate test to diagnose this patient’s condition?