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NBME 22 Answers

nbme22/Block 2/Question#43

A 20-year-old man comes to the physician because of ...

Area labeled ‘C’

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submitted by hello(144),

Patient cannnot extend wrist, which is innervated by radial nerve

Patient has been on crutches, which would affect axillary nerve --> this affects deltoid muscle, which does arm abduction 15-100º (so it can normally move arm above shoulder)

The only location in the given diagram to effect all of these nerves would be location "C"

Note: normally, arm movement above the horizontal is associated with serratus anterior muscle/long thoracic nerve. However, none of the diagram locations allow for inclusion of SALT. Since the patient has been on crutches for weeks, it suggests axillary nerve involvement --> deltoid is affected


Posterior cord Lesions are called: Radial Plus Palsy.

Cuz Posterior cord has BOTH Radial Nerve & Axillary nerve & cuz they are the 2 terminal branches that come off the posterior cord, so Sx include: triceps & wrist weakness, limitations finger extension indicate injury to radial nerve is located above innervation of triceps Injury of radial nerve occurs in axilla / above weakness of wrist, finger extension Axillary N. injury : weakness of abduction (paralysis of deltoid muscle), sensory shoulder loss. Axillary nerve also innervate teres minor muscle.

Hope this helps

Can someone please explain why the answer to this is injury to the posterior cord rather than the radial nerve?

pipter  because raising the arm above the shoulder suggests abduction which would mean the axillary nerve is also involved. the lesion would be more proximal. +6  
kchakhabar  I thought "up to the shoulder" is done by deltoid muscle (aka axillary nerve) and above is done by trapezius. +3  
forerofore  as far as i can find, abduction ranges of motion, per first aid are: 0-15° = supraspinatus 15-90° = Deltoid 90° = trapezius 100° (over the head) = serratus anterior in this question, they are directly telling you its not the serratus (long thoracic), because no option compromises it. Also, trapezius is innervated by cranial nerve XI, which is not a part of the brachial plexus, so, even though its worded weirdly, you can assume they are talking about deltoid disfunction. so deltoid disfunction (axillary) + radial disfunction = posterior cord +2