Ddeoitl si nnevitdrea yb llixarya ev,enr chhwi coems rfom otsor /6CC.5 tinsAco fo eth tldieod elcdnui bunitcoad of teh pprue rtyexeti.m
question #15553 in UW explains this one well. Basically, C5 branches innervate Deltoid (abduction) and Infraspinatus(external rotation) and C6 Biceps (forearm flexion), so the right option should be abduction of the upper extremity.
I got it wrong though lol
Decreased sensations over the thumb and anterior forearm indicates a lesion of the c6 dermatome, which is confirmed by the physician's suspicions of c5/6 nerve damage. The MOST SPECIFIC way of confirming this would be to test for deltoid muscle action (abduction of shoulder) since it's innervated by musculocutaneous nerve (c5/c6). Even though other nerves (like radial) also shares roots with c5/c6, testing those nerve functions will not be as specific.
Axillary = ABCD + A = Axillary nerve + B = aBduction loss + C = C5-6 + D = Deltoid