A first-time mоther experiences а quick lаbоr аnd delivery. The Obstetrician used the baby's left arm tо help in the delivery. At the child's ninth 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 for arm abduction with weak supination of the forearm. What is the most probable diagnosis and which roots are involved?