McArdle Disease: Insights Into a Rare Metabolic Myopathy in a Young Boy With Recurrent Exercise-Induced Muscle Weakness.

McArdle disease, or glycogen storage disease type V, is a rare metabolic myopathy caused by a deficiency of myophosphorylase, leading to impaired glycogen breakdown in skeletal muscle. This results in exercise intolerance, muscle cramps, and episodic weakness. Due to its rarity and variable presentation, diagnosis is often delayed or misattributed to other neuromuscular disorders. We report the case of a nine-year-old boy who presented to the emergency department with acute difficulty in walking associated with left hip pain. He had a prior episode at the age of five with similar complaints, during which routine hematological and biochemical investigations, including creatine kinase (CK) levels, were unremarkable. He was initially diagnosed with viral myositis and managed conservatively. In the current episode, a physical examination revealed proximal muscle weakness with tenderness over the left hip. Laboratory investigations showed a markedly elevated CK level of 1390 IU/L, suggesting muscle pathology. Imaging studies, including hip ultrasonography and lumbosacral spine MRI, were normal. Given his clinical history and episodic muscle weakness, whole-exome sequencing was performed, which confirmed pathogenic variants in the PYGM gene, establishing the diagnosis of McArdle disease. The patient was referred to a dietitian and placed on a structured nutritional and exercise regimen, leading to significant symptomatic improvement. This case highlights the importance of considering McArdle disease in children presenting with recurrent exertional muscle weakness and myalgias, particularly in the setting of elevated CK levels. Early recognition and genetic confirmation are crucial to prevent complications and guide appropriate dietary and exercise-based interventions. Increased awareness among clinicians can facilitate timely diagnosis and improve long-term outcomes in affected individuals.