A first-time mother experiences a quick labor and delivery….

A first-time mother experiences a quick labor and delivery. The Obstetrician used the baby’s left arm to help in the delivery. At the child’s second month check-up with the pediatrician, it is realized the baby has a significant decrease in muscle tonicity in the left arm compared to the right. A medical examination of the infant showed the inability to flex the elbow with weak supination of the forearm. What is the most probable diagnosis and which roots are involved?