Morquio syndrome masquerading as juvenile idiopathic Arthritis: A case report.

Morquio syndrome, or Mucopolysaccharidosis type IV (MPS IV), is a rare autosomal recessive lysosomal storage disorder caused by enzyme deficiencies involved in glycosaminoglycan degradation. Progressive accumulation of these molecules leads to skeletal dysplasia, joint deformities, and variable systemic complications. Early features may mimic juvenile idiopathic arthritis (JIA), delaying diagnosis. We report a 16-year-old adolescent previously diagnosed with polyarticular JIA and treated with biological therapy since age 7. He presented with generalized polyarthralgia, significant skeletal deformities, and a 6 cm leg length discrepancy. Clinical examination revealed hand deformities, thoracic cage enlargement, and spinal involvement. Radiographs demonstrated dysplastic femoral heads and shoulder deformities. Laboratory tests were negative for autoimmune markers. Urinary glycosaminoglycan analysis and targeted genetic testing of the glucosamine N-acetyl-6-sulfatase (GALNS) gene confirmed Morquio syndrome. The patient was referred for genetic counseling and orthopedic management. This case highlights the diagnostic challenge of differentiating MPS IV from JIA, particularly in patients with atypical skeletal features or poor response to immunomodulatory therapy. Recognition of radiological abnormalities, family history, and consanguinity is essential. Early diagnosis allows for appropriate enzyme replacement therapy, surgical interventions, and multidisciplinary care to optimize outcomes. Morquio syndrome can masquerade as polyarticular JIA. High clinical suspicion, combined with enzymatic and genetic testing, is crucial for timely diagnosis and management to improve long-term function and quality of life.