Use the clinical features of McArdle disease, the “second wind” phenomenon in McArdle disease, and the laboratory findings in McArdle disease to frame the working diagnosis and next step; use it only when it changes interpretation or management. Make the “second wind” phenomenon in McArdle disease the checkpoint that determines whether you escalate testing, narrow the differential, or change treatment.

McArdle disease usually begins with exertional cramps and myalgias in adolescence or early adulthood; The second-wind phenomenon reflects a useful metabolic shift after brief rest; Forearm exercise testing classically shows no lactate rise with preserved ammonia response; Biopsy should be delayed after an attack to avoid a falsely normal result; Carbohydrate strategies and pacing help exercise tolerance, while explosive exertion should be avoided