Gastric Melanoma in A 19-Year-Old Patient: A Rare Case and A Review of Diagnostic Pitfalls.

Background: Gastric melanoma is exceptionally rare, typically metastatic and found in older adults. Primary cases are even rarer. We present a novel case in a 19-year-old male, underscoring unique diagnostic challenges due to his youth and lack of a known primary lesion. Case description: A previously healthy 19-year-old male presented with postprandial vomiting, weight loss, constipation, and fatigue. Endoscopy revealed multiple pigmented gastric nodules. Biopsies confirmed malignant melanoma via immunohistochemistry (S100, HMB45, Melan A positive; cytokeratin, CD117, DOG1 negative). Imaging showed widespread metastases (hepatic, pulmonary, osseous). The patient deteriorated rapidly and died before a primary lesion could be identified. An autopsy was not performed. Discussion: This case presents a profound diagnostic dilemma. The patient's age and absence of a primary lesion are highly atypical for metastatic melanoma. While the metastasis burden suggests a secondary origin, a primary gastric source could not be ruled out. This underscores that melanoma must be considered in the differential diagnosis of pigmented gastric lesions in all age groups. Immunohistochemistry is indispensable for confirming the diagnosis and excluding more common malignancies when faced with an atypical clinical presentation. Conclusion: This report describes the youngest documented patient with gastric melanoma. It highlights the critical need to include melanoma in the differential diagnosis of young patients with gastrointestinal symptoms and endoscopic findings of pigmented lesions. The case emphasizes the aggressive potential of this disease and the essential role of immunohistochemical staining for timely and accurate diagnosis. LEARNING POINTS: Gastric melanoma, while rare, must be considered in the differential diagnosis of young patients presenting with non-specific gastrointestinal symptoms and pigmented endoscopic lesions, as it can occur in the absence of classic risk factors.A definitive diagnosis relies on a specific immunohistochemical panel (S100, HMB45, Melan-A) to confirm melanocytic origin and rule out more common gastric malignancies like carcinoma or gastrointestinal stromal tumour.This malignancy can be exceptionally aggressive and progress very rapidly in young adults, underscoring the critical importance of a prompt diagnosis and urgent intervention to improve outcomes.