Incidental Finding of Unilateral Tensor Fascia Lata Agenesis in a Marathon Runner: An Unreported Phenomenon.

Congenital agenesis of the tensor fascia lata (TFL) muscle is an extremely rare anomaly, with very few reports in the literature and unclear clinical significance. We report the incidental finding of unilateral TFL agenesis in a 25-year-old male physician who had been enrolled as a healthy control in a muscle MRI study on genetic myopathies. Imaging demonstrated a complete absence of the right TFL with mild compensatory hypertrophy of the ipsilateral rectus femoris, while the contralateral side and all other muscles appeared normal. The subject had no history of neuromuscular disease, exhibited only a subtle waddling gait, and had previously completed the New York Marathon in 4 h and 16 min without symptoms. Laboratory tests, including creatine kinase, were within normal limits. Thirteen years later, he remains in good health, continues regular sports activities, and has not developed pain or functional impairment. This case emphasizes that TFL agenesis may remain clinically silent and compatible with high levels of physical activity. Nevertheless, awareness of such anomalies is important, as compensatory mechanisms might predispose to long-term biomechanical imbalance, and recognition on imaging can prevent misinterpretation or unnecessary investigations.