Adult-Onset Fanconi Syndrome Presenting as Stress Fractures: A Case Report.

Introduction: Fanconi syndrome is a disorder characterized by defective proximal renal tubular reabsorption, resulting in the wasting of glucose, phosphate, bicarbonate, amino acids, and uric acid. While most common in childhood and usually secondary to other conditions in adults, idiopathic cases with adult onset are rare. Skeletal complications such as osteomalacia and fractures may result from chronic phosphate loss and can be the initial presentation in adult cases. Case Report: A 35-year-old previously healthy man presented with chronic back pain and bilateral hip pain, later diagnosed as bilateral femoral neck stress fractures. Laboratory evaluation revealed hypophosphatemia, renal glucosuria with normoglycemia, aminoaciduria, phosphaturia, tubular proteinuria, and proximal renal tubular acidosis. Secondary causes were excluded, and genetic testing was not feasible, leading to a diagnosis of idiopathic adult-onset Fanconi syndrome. The patient was treated with sodium bicarbonate and potassium citrate, resulting in improvement and prevention of further fractures. Conclusion: This case demonstrates that adult-onset Fanconi syndrome, though rare, can initially present with musculoskeletal symptoms such as stress fractures. Early recognition and interdisciplinary evaluation are essential for timely intervention, which may prevent further complications even in the absence of an identifiable cause.