Wednesday, May 24, 2017

NBME 7 question on muscle weakness

Disclaimer: This is an NBME form 7 question for step 2 CK. If you are planning to take USMLE step 2 CK in the future, I would recommend that you DO NOT read this post because it will bias your assessments.
37 yo M - progressive weakness over the last 5 days, began at his ankles, then went on to involve the proximal muscles of the lower extremities, hands, forearms, and face. Mild tingling of fingers and feet. Had an URI 2 weeks ago which is not present now. On exam: moderate weakness of facial muscles, proximal muscles of extremities, and marked weakness of distal muscles of extremities. DTRs are absent. Hypotonia of knees and ankles. Sensation to light touch and vibration is mildly decreased over the feet. Most likely cause?

A. Decreased acetylcholine receptors
B. Decreased release of acetylcholine from motor nerve terminals
C. Degeneration of muscle fibers
D. Demyelination of axons
E. Loss of motor neurons

I think this is D, I got E wrong. I feel stupid!

Acute Inflammatory Demyelinating Polyradiculoneuropathy (AIDP- Guillain-Barré Syndrome)
Acute inflammatory demyelinating polyradiculoneuropathy (AIDP) is a primarily demyelinating neuropathy with autoimmune etiology. Motor axons are affected more than sensory axons. Incidence in children is similar to that seen in adults. Children often have a prodromal respiratory or gastrointestinal infection occurring within one month of onset. Common precipitating infections include Mycoplasma, cytomegalovirus, Epstein-Barr virus, Campylobacter jejuni, and various vaccinations. Weakness generally begins distally in the lower extremity with a progressive ascending paralysis ultimately involving the upper limbs. Pain and sensory symptoms are not uncommon. The most common cranial nerve abnormality is an ipsilateral or bilateral lower motor neuron facial paralysis. Objective sensory loss has been documented in the minority of children.

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