A 74-year-old man with blood cancer developed a serious fungal skin infection caused by Mucorales while taking multiple targeted cancer drugs. The infection appeared as a darkening ulcer on his ankle that rapidly worsened. Doctors diagnosed it using a skin biopsy showing characteristic fungal structures, though standard fungal tests came back negative. The patient required amputation of his leg below the knee to remove all the infected tissue, along with antifungal medications, and ultimately recovered.