Motor weakness in the deltoids is indicative of which spinal nerve root?

Motor weakness in the deltoids primarily points towards an issue with the C5 spinal nerve root. The deltoid muscle, responsible for shoulder abduction, is innervated mainly by the axillary nerve, which is a terminal branch of the C5 nerve root. A lesion or dysfunction at this level can lead to weakness or inability to elevate the arm at the shoulder, which is a hallmark of deltoid involvement.

Understanding the dermatomes and myotomes can further assist in identifying neurological issues. The C5 nerve root also contributes to elbow flexion as part of the musculocutaneous nerve function, but the specific weakness in the deltoids directly correlates with the loss of motor function linked to C5.

Considering the other options, C6 and C7 are more associated with functions like wrist flexion and finger extension, while L4 is related to movements involving the quadriceps and knee extension. Therefore, the clear association of the C5 root with the deltoids makes it the correct answer.