An Unusual, Symmetric Rash on Both Forearms in a 94-Year-Old Patient A Clinicopathological Challenge and Diagnostic Pitfall.

Trichophyton rubrum (T. rubrum), an anthropophilic ectothrix dermatophyte usually causes minor skin infection confined to the stratum corneum of the epidermis, to nails and the hair in immunocompetent persons. Life-threatening invasive dermatophyte infections are rare and mainly reported in immunosuppressed patients such as organ transplant recipients. We herein present an extraordinary case of a symmetric bilateral invasive T. rubrum infection on both forearms in a 94-year-old male patient, histomorphologically mimicking cutaneous blastomycosis. Invasive T. rubrum with atypical large round spore-like organisms resembling Blastomyces dermatitidis (B. dermatitidis) is common diagnostic pitfall but has never been reported in the setting of a concomitant systemic anti PD-1 checkpoint-inhibitor treatment. Correct diagnosis of atypical Blastomyces-like T. rubrum based on morphology and staining characteristics alone can be challenging and emphasizes the need for confirmation via culture or PCR-testing.