Final answer:
The injury to the ventral rami of the brachial plexus that would account for the infant's arm presentation, with it being adducted, medially rotated, and the forearm extended and pronated, is to the C5 and C6 fibers, affecting the muscles responsible for flexing and supinating the forearm.
Step-by-step explanation:
The symptoms described in the infant, characterized by limited movement of the right upper limb, the arm being adducted and medially rotated, and the forearm being extended at the elbow and pronated, are indicative of an injury to the upper brachial plexus. This specific pattern of arm positioning is often referred to as "waiter's tip" position and is typically associated with an injury involving the C5 and C6 fibers in the ventral rami of the brachial plexus. These nerve roots contribute to the formation of the muscles responsible for flexing and supinating the forearm, namely the biceps brachii, brachialis, and brachioradialis. When these nerves are injured, the muscles they innervate cannot function properly, leading to the observed posture of the limb. Additionally, injuries to the brachial plexus can be associated with fractures of the humerus, which may compromise blood flow and nerve function in the limb.