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NBME 22 Answers

nbme22/Block 3/Question#1
A 35-year-old man comes to the physician ...
C7 nerve root🔍

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 +3  upvote downvote
submitted by mcl(453),

Pronator teres and quadratus are both supplied by median nerve (C5/C6/C7/C8/T1, so that's not super helpful.) Extension of the forearm is radial nerve (also C5-T1, also not helpful). This does tell us is it can't be isolated median or radial. Triceps tendon reflex is C7/C8, which narrows it down to these two.

Can anyone explain why it's C7 over C8?

joha961  Same question. How could you determine between the specific nerve roots (C7 vs. C8)? +2  
mcl  Someone I was talking to (and post below) was saying that first aid mentions triceps is C7, so that's what should've been the big thing for us. +1  
joanmadd  you might see some ulnar nerve involvement if C8 was involved her +  

 +3  upvote downvote
submitted by paulm(3),

Per UWorld (ID 15553)

The C7 root is the most frequently affected nerve root in patients with cervical radiculopathy, resulting in deficits across the median and radial nerve distributions.

"tingling sensation of fingers" → median nerve

"triceps muscle reflex is decreased" → radial nerve

carmustine  Another important indication that this involves the median nerve is the weakness in pronation. Pronation of the forearm is performed by the pronator teres and the pronator quadratus, both of which are innervated by the median nerve. +1  

 +2  upvote downvote
submitted by delamj(3),

My thought process on this question:

  • weakness of arm extension and pronation = sounds like musculocutaneous nerve (C5-C7)

  • Triceps stretch reflex (C7-C8);

Ruleouts: Biceps is C5-C6 so r/o C6. C8 doesn't fit musculocutaneous muscle actions action so r/o C6. Median and Radial nerves have different muscle actions so r/o those as well, leaving just C7 best fitting the vignette.

delamj  C8 doesn't fit musculocutaneous muscle actions action so r/o C8.** +  

According to FA 2019 Tricep Reflex is mediated by C6, yes C6 NOT C8, and C7 [In BOLD, implying it is more important].

peridot  This is on p.498 of FA 2019 for anyone curious +  

 +1  upvote downvote
submitted by welpdedelp(154),

So I think that issue of wrist extension and/or finger drop would be more radial nerve. However, there was more proximal weakness, so it would be C7.

"7-8 lay them straight", the pt couldn't "lay them straight" so it would be C7 root

welpdedelp  *As an addition, median nerve involvement would have leaned more toward C8 than C7. +1  
meningitis  Do you have anymore useful mnemonics for brachial plexus? +  
henoch280  FA pg 494 for mnemonics +  
winelover777  Doesn't look like there are many in FA 2019. S1/S2 - Buckle my shoe. L3/L4 - Shut the door. C5/C6 - Pick up sticks. +  
drzed  S2-S4 keeps the penis off the floor :) (cremaster reflex) +  
peridot  What's crazy @drzed is that in FA 2019 it says L1-L2 ("testicles move") on p.498 so I wonder if that changed +  

According to Uworld, QID: 11779 C7 myotome is involved in elbow extension. Looks like it settles!

 +0  upvote downvote
submitted by nwinkelmann(218),

Per Wikipedia: The triceps reflex, a deep tendon reflex, is a reflex as it elicits involuntary contraction of the triceps brachii muscle. It is initiated by the Cervical (of the neck region) spinal nerve 7 nerve root (the small segment of the nerve that emerges from the spinal cord). The reflex is tested as part of the neurological examination to assess the sensory and motor pathways within the C7 and C8 spinal nerves.

Also, from a different website: The triceps reflex is mediated by the C6 and C7 nerve roots, predominantly by C7. https://informatics.med.nyu.edu/modules/pub/neurosurgery/reflexes.html.

this is my way to think(please correct me if it's not correct: a. weakness of triceps muscle reflex --> C6 / C7 / C8 problem (FA P498 2019) b. weakness of extension of the forearm --> radial nerve is involved --> damage of C7 root leads to axillary or RADIAL nerve defect (FA P441 2019)

+/- joyceeepan(3),

I’m confused about this one. Weakness of extension and pronation of the right forearm, with a decreased triceps muscle stretch reflex. The answer was C7 nerve root, but how are you able to localize to C7 and not C6 or C8? I figured that extension is radial, C5-T1, and pronation is median, also C5-T1. Triceps reflex is C7-C8 (FA2019 says C6-C7). How would you narrow down to just C7 damage?

txallymcbeal  My FA2018 has “C7” bolded, meaning it is the main nerve root. But I also got this one wrong so I can’t be much help besides that. +1  
mnemonia  Honestly just a guess but I have this vague understanding that intrinsic hand muscles are C8-T1 so we might’ve expected more hand motor findings as well with a C8 lesion. +2  
theecohummer  I narrowed it down to C7 using the fact that the C7 myotome is elbow extension. I also learned that the C7 nerve root was the main contributor to the triceps DTR so I just went with that. +  
mchu21  They also mentioned that the person had weakness pronating the right forearm which is performed by the biceps. Biceps is innervated by the musculocutaneous nerve which is C5-C7 and that's what helped me pick C7 > C8. +1  
mcl  Sorry, I thought the biceps was a supinator of the forearm? +5  
henoch280  yes.. its the supinator not pronator +  

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