A 47-year-old man with melanoma and a history of papillary thyroid cancer treated with thyroidectomy presented with 1 week of low-grade fevers, myalgias, and proximal muscle weakness. Three months earlier, he underwent digital amputation of his right third finger and began adjuvant nivolumab treatment for stage IIIC acral-lentiginous melanoma. One week later, he developed fevers, myalgias, dark urine, weakness, and difficulty walking. He had no palpitations, chest pain, or shortness of breath.

On examination, his blood pressure was 135/86 mm Hg, pulse 78/min, and he was afebrile. He had reduced strength in bilateral proximal shoulders (3/5), elbow flexors/extensors (3/5), and forearm flexors/extensors (4+/5). No rashes were noted.