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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.<\/jats:p>","DOI":"10.1136\/bcr-2025-270035","type":"journal-article","created":{"date-parts":[[2026,4,24]],"date-time":"2026-04-24T14:30:23Z","timestamp":1777041023000},"page":"e270035","update-policy":"https:\/\/doi.org\/10.1136\/crossmarkpolicy","source":"Crossref","is-referenced-by-count":0,"title":["Acral lentiginous melanoma presenting as an enlarging axillary mass"],"prefix":"10.1136","volume":"19","author":[{"ORCID":"https:\/\/orcid.org\/0009-0003-4077-5468","authenticated-orcid":false,"given":"Lily","family":"Rajaee","sequence":"first","affiliation":[{"name":"Medicine, Loyola University Chicago Stritch School of Medicine, Maywood, Illinois, USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Yingjoy","family":"Li","sequence":"additional","affiliation":[{"name":"University of California Irvine School of Medicine, Irvine, California, USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Warren","family":"Chow","sequence":"additional","affiliation":[{"name":"University of California Irvine Division of Hematology\/Oncology, Orange, California, USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Thuy B","family":"Tran","sequence":"additional","affiliation":[{"name":"Surgical Oncology, University of California Irvine, Orange, California, USA"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"239","published-online":{"date-parts":[[2026,4,24]]},"reference":[{"key":"2026042407301650000_19.4.e270035.1","first-page":"9","article-title":"Cancer statistics, 2014","volume":"64","author":"Siegel","year":"2014","journal-title":"CA A Cancer J Clinicians"},{"key":"2026042407301650000_19.4.e270035.2","doi-asserted-by":"publisher","DOI":"10.3322\/caac.21820"},{"key":"2026042407301650000_19.4.e270035.3","doi-asserted-by":"crossref","DOI":"10.3390\/biom15010120","article-title":"Acral Melanoma: A Review of Its Pathogenesis, Progression, and Management","volume":"15","author":"Kim","year":"2025","journal-title":"Biomolecules"},{"key":"2026042407301650000_19.4.e270035.4","doi-asserted-by":"publisher","DOI":"10.1111\/pcmr.12885"},{"key":"2026042407301650000_19.4.e270035.5","doi-asserted-by":"crossref","first-page":"354","DOI":"10.1038\/s41586-022-04584-6","article-title":"Anatomic position determines oncogenic specificity in melanoma","volume":"604","author":"Weiss","year":"2022","journal-title":"Nature New Biol"},{"key":"2026042407301650000_19.4.e270035.6","doi-asserted-by":"publisher","DOI":"10.1067\/mjd.2003.63"},{"key":"2026042407301650000_19.4.e270035.7","doi-asserted-by":"crossref","first-page":"431","DOI":"10.1111\/pcmr.13090","article-title":"Undereducation is afoot: Assessing the lack of acral lentiginous melanoma educational materials for skin of color","volume":"36","author":"Elad","year":"2023","journal-title":"Pigment Cell Melanoma Res"},{"key":"2026042407301650000_19.4.e270035.8","doi-asserted-by":"crossref","unstructured":"Fariyike O , Johnson NA , Craig P , et al . 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