NBME 18 Myelinated primary afferents (NBME Answers)

Retired NBME 18 Answers

nbme18/Block 3/Question#41 (reveal difficulty score)
24 yo man, 3 days of progressive numbness of ...
Myelinated primary afferents 🔍 / 📺 / 🌳 / 📖
tags: neuro repeat

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submitted by ∗spow(50)

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Here's how I thought through this. problem with DCML (absent proprioception and vibration sense), problem with deep tendon reflexes (DRGs), ataxic gait (spinocerebellar pathway), mild weakness (motor neurons). The only thing that all of these pathways have in common is that they all use myelinated afferents.

I don't know if Guillan Barre would actually present like this, but you don't have to know what the illness is to figure the question out.

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queestapasando Might be acute inflammatory demyelinating polyradiculopathy (FA 2019 p.512): "Most common subtype of Guillain-Barré syndrome. Autoimmune condition that destroys Schwann cells via inflammation and demyelination of motor fibers, sensory fibers, peripheral nerves..." +5

md_caffeiner sounds really great, and i thought the same and answered the same way. but while in the exam, i had that "how THE FUCK could motor weakness be an afferent??" moment. quite frequently forget the fact that this is an nbme full of mess. lol. +6

2059nyc It could be explained by the lack of the sensory component for the DTR and not the motor neurons? +2

sonofarathorn I focused on the loss of vibration and joint position sensation. Pacinian corpuscles sense vibration. Meissner corpuscles and Merkel discs sense position. These sensory receptors are all large myelinated afferents FA 2019 p.482 +2

mgadda the question asks the most likely explanation for the SENSORY findings +6

aaftabsethi1 Can anyone explain numbness and tingling ? Isnt that through unmyelinated fibres? +1

cassdawg @aaftabsethi unmyelinated afferents include fibers that carry: slow pain, heat, and olfactory senses. +

GBS is an inflammatory demyelinating polyneuropathy affecting motor and sympathetic efferent fibers.
Sensory afferent fibers are also affected.
It presents with acute, ascending bilateral muscle weakness, sensory loss, and Hyporeflexia.

+1/- namesthegame22(14)

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submitted by ∗sympathetikey(1600)

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Guillain Barre is T-cell mediated destruction of myelinated axons. Generally presents with a rapid onset following a viral / bacterial illness with ascending paralysis.

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jbrito718 Autoantibodies are against GM1 gangliosides on myelin! +

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submitted by ∗amirmullick3(76)

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Is this Guillain barre though? I felt it was Acute inflammatory demyelinating polyradiculopathy, discussed on page 512 FA 2019.

There is no relation to C jejuni here nor does the patient have any other relations to infection such as eating something or etc.

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wishmewell Acute inflammatory demyelinating polyradiculopathy is a subtype of GBS. +7

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submitted by ∗godby(2)

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Motor + Sensory + rapid progress :: may be R/O Acute inflammatory demyelinating polyradiculopathy

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