Primary anorectal melanoma with BRAF V600 wild-type status presenting with widespread metastases in a 29-year-old man: a case report

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Abstract Primary anorectal malignant melanoma (ARMM) is a rare and highly aggressive mucosal neoplasm, accounting for <1% of all melanomas. Typically affecting older patients, its presentation in young adults is uncommon and often causes diagnostic delays. We report a 29-year-old man presenting with hematochezia, tenesmus, and post-defecatory pain. Proctoscopy revealed a friable, ulcerated, hemi-circumferential exophytic mass at the dentate line. Biopsy demonstrated a high-grade, undifferentiated, extensively necrotic neoplasm with focal intracytoplasmic pigment. Immunohistochemistry confirmed melanocytic lineage (diffuse S100/SOX10; focal HMB-45/Melan-A). Molecular analysis identified wild-type BRAF V600 status. Staging revealed extensive nodal, pulmonary, hepatic, adrenal, and peritoneal metastases; occult primary sites were excluded. The patient received palliative systemic therapy. This case underscores ARMM’s aggressive clinical course, emphasizing the need to consider melanoma for undifferentiated tumors in young adults, alongside the essential role of comprehensive immunohistochemical and molecular profiling for accurate diagnosis.