You are asked to evaluate an infant born vaginally 3 hours previously to a mother whose only pregnancy complication was poorly controlled insulin dependent gestational diabetes. The nursing staff noticed that the infant was breathing abnormally. On examination, you find that the infant is cyanotic, has irregular, labored breathing, and has decreased breath sounds on the right side. You also note decreased tone in the right arm. You provide oxygen and order a stat portable chest radiograph, which is normal. Which of the following studies is most likely to confirm your diagnosis?

Infants born to mothers with gestational diabetes are at risk for being large for their gestational age and thus at increased risk for peripheral nerve injuries such as Erb-Duchenne and phrenic nerve paralysis. An ultrasound or fluoroscopy of the chest would reveal asymmetric diaphragmatic motion in a seesaw manner. While a chest film can be normal as in this case, an elevated hemidiaphragm may be observed. With a negative chest radiograph, a chest CT would not be helpful at this point. Bronchoscopy would help delineate airway abnormalities and foreign bodies, but would not identify phrenic nerve paralysis.