PAED-0210

A 5 month old girl was admitted to the emergency department with reduced movement of her right arm. History reveals she was born at term, 4.5 kg and via natural delivery. Her mother reports her head was stuck for a while before she was fully delivered. Postnatal examination revealed reduced movement of her right arm as well but was thought to improve over time. The baby cannot sit and her right arm is flaccid, internally rotated, adducted, and extended. The reflexes are absent in the right arm. Which of the following investigations would provide the most useful information?