Trichosporon asahii is an emerging opportunistic fungus usually associated with invasive infections in immunocompromised patients, particularly fungemia. However, its presentation as primary osteomyelitis is extremely rare, especially in individuals without significant immunosuppression.

This report describes an unusual case of T. asahii osteomyelitis in an elderly, previously healthy patient, reinforcing the importance of early diagnosis and aggressive therapeutic management in atypical invasive fungal infections. An 83-year-old male with only systemic arterial hypertension was admitted with pain, edema, and erythema of the right lower limb. He reported a lateral malleolar ulcer on the right foot for two months, without prior treatment. Laboratory tests showed leukocytosis and a marked increase in C-reactive protein. He was initially treated with ceftriaxone and clindamycin for 14 days, with partial improvement of the skin findings but worsening of the ulcer. MRI revealed signs of malleolar osteomyelitis. Surgical debridement was performed and deep tissue was collected for culture. Identification by MALDI-TOF and sequencing of the ITS region confirmed T. asahii, and direct examination demonstrated septate hyphae and blastoconidia. Intravenous voriconazole was started, followed by transition to oral therapy, maintained for six months. Treatment was combined with negative pressure wound therapy, dermal matrix implantation, and surgical grafting, leading to hospital discharge after nine weeks. The patient has had no clinical recurrence to date.

This case expands the clinical spectrum of T. asahii, demonstrating its ability to cause invasive osteoarticular infection even in the absence of immunosuppression. Intrinsic resistance to amphotericin B, fluconazole, and echinocandins highlights voriconazole as the antifungal of choice. The combination of accurate microbiological diagnosis, prolonged antifungal therapy, and extensive surgical management was decisive for therapeutic success. In the context of increasing invasive mycoses, updated clinical-laboratory protocols are needed to support early recognition and appropriate management of emerging fungal infections.