**Question:** On exercise testing, a patient with McArdle disease exhibits:

**Core Concept:** McArdle disease is a rare genetic disorder affecting the skeletal muscles, caused by a deficiency in the enzyme myophosphorylase. This leads to impaired glycogen breakdown during intense physical activity, causing characteristic symptoms like severe muscle pain and fatigue.

**Why the Correct Answer is Right:** In McArdle disease, the deficiency of myophosphorylase enzyme results in the inability to break down glycogen during intense physical activity. This leads to a buildup of inorganic phosphate and reduced ATP production in the muscles, causing the characteristic symptoms seen in patients with McArdle disease.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because exercise testing does not reveal the presence of myophosphorylase deficiency, which is the pathophysiological basis of McArdle disease. Exercise testing will instead show symptoms related to impaired glycogenolysis.

B. This option is incorrect because exercise testing focuses on evaluating the patient's response to physical activity, which is impaired in McArdle disease due to myophosphorylase deficiency.

C. This option is incorrect because exercise testing in McArdle disease primarily demonstrates the patient's inability to break down glycogen and generate ATP during intense physical activity.

D. This option is incorrect because, in McArdle disease, exercise testing reveals the impaired ability to break down glycogen and produce ATP during intense physical activity, leading to characteristic symptoms.

**Clinical Pearl:** Exercise testing in McArdle disease highlights the impaired glycogenolysis and ATP production during intense physical activity. This knowledge aids in understanding the pathophysiology of the disease and differentiating it from other muscle-related conditions with similar symptoms.