Acral lentiginous melanoma presenting as an enlarging axillary mass

Acral lentiginous melanoma (ALM) is a rare subtype of melanoma skin cancer that disproportionately affects underrepresented minorities and is associated with poor prognosis. We report a case of a middle-aged Hispanic woman presenting to the emergency department with an enlarging, painful axillary mass. The initial differential diagnosis focused on occult breast carcinoma, but comprehensive workup with breast imaging was negative. Careful physical examination revealed an undiagnosed pigmented lesion of the left thumb. Excisional biopsy of the left nail unit demonstrated malignant melanoma. Ultrasound-guided core biopsy of left axillary mass confirmed metastatic malignant melanoma. The patient received neoadjuvant immunotherapy followed by left thumb amputation and axillary lymph node dissection. This case illustrates a clinical scenario where melanoma was an unexpected diagnosis that was not in the initial differential diagnoses, underscoring the importance of comprehensive physical examination of non-sun-exposed extremities to facilitate timely diagnosis and management.