NBME 16 Answers ↦
Going off that, if there's a stem with a patient getting tired quickly during exercise, McArdle should be on the differential. It's an older dude who APPARENTLY never experienced this before but w/e. McArdle is associated w/ glycogen phosphorylase (AKA myophosphorylase) problems
The question stem also mentions that there is no rise in lactic acid. This could be because if we cannot even break down glycogen to glucose to get pyruvate, we can't even turn pyruvate to lactate via lactate dehydrogenase. And also, without anaerobic metabolism, you fatigue quickly.
FA 2020 pg. 87 also says there is a normal rise in ammonia levels during exercise in McArdle patients. They also suffer from arrhythmias from electrolyte abnormalities.
There is also a "second wind phenomenon" where they are able to tolerate exercise better after getting more blood flow to their muscles. This could be due to glucose in bloodstream bringing glucose to muscles?