USMLERx is hard.

[auburnO5]

I've been doing blocks of 48 with "hard" questions. And they are hard.

I'm averaging about 85-90% on UW, the last NBME I took predicted me at 254, yet I can barely get 60% on USMLERx. And it has my predicted score at "224." Way too hard.

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[MrBeauregard]

I finished Kaplan a couple of weeks ago, and I distinctly remember a question like that. The patient had a history of asthma and was presenting with proximal muscle weakness. Since corticosteroids are the first-line treatment for chronic asthma management, it stands to reason that the weakness was due to chronic steroid use. The diagnosis in question ended up being steroid myopathy, but the actual answer choice was 'muscle biopsy showing selective atrophy of type II fibers'. The patient had proximal muscle weakness due to chronic steroid use but actually didn't have Cushing's due to the absence of other classic Cushingoid features. The physical exam also yielded normal muscle mass with normal stretch reflexes.

 

[Phloston]

Any thoughts as to the type-II-selective atrophy?

 

[MrBeauregard]

Nope. A PubMed search yielded fairly consistent results: selective type II fiber atrophy with only type I fibers showing necrosis. None of the articles [could] explain why the atrophic changes were type II-selective, though.

 

[Phloston]

The only thing I can reasonably think of would be that because type-II muscle is the fast-twitch type, it would have a lesser mitochondrial density per unit mass of protein, so it would therefore be less metabolically expensive to utilize amino acids from it during gluconeogensis as compared with the slow-twitch type.

 

[MrBeauregard]

Absolutely. This patient, through his daily use of corticosteroids, has essentially put himself in a chronic state of gluconeogenesis. Since type I fibers have higher mitochondrial density and thus utilize aerobic respiration, stealing amino acids from these fibers wouldn't be favorable, as they are needed in the TCA cycle. As a result, amino acids are stolen from type II fibers, which have less mitochondria and can effectively utilize anaerobic respiration for their energy needs, thus leading to type II selective atrophy.