<![CDATA[Introduction: Lynch syndrome, (LS), or hereditary nonpolyposis colorectal cancer (HNPCC), is an autosomal dominant inherited cancer syndrome primarily caused by germline mutations in DNA mismatch repair (MMR) genes, particularly MLH1 and MSH2. These mutations cause microsatellite instability (MSI), a hallmark of LS-associated colorectal cancer. Despite its clinical significance, LS remains underdiagnosed, particularly in low-resource settings. Case Presentation: A 36-year-old man presented with intermittent right upper abdominal pain, chronic diarrhea, and unintentional weight loss of 17 kg over three months. Colonoscopy revealed an ascending colon mass, while computed tomography showed a malignant lesion causing luminal narrowing and mesenteric fat infiltration. Histopathology confirmed high-grade mucinous adenocarcinoma. Immunohistochemistry showed loss of MLH1 and PMS2 expression with retained MSH2 and MSH6, indicating MMR deficiency and MSI-high status. A detailed pedigree identified six first- and second-degree relatives with colorectal cancer, fulfilling the Amsterdam II and revised Bethesda criteria for LS. The patient underwent right hemicolectomy followed by adjuvant FOLFOX chemotherapy. Immunotherapy could not be administered because of limited access despite its established benefit in MSI-high colorectal cancer. Conclusions: This case highlights the importance of integrating clinical presentation, family history, and molecular findings in diagnosing LS. MMR immunohistochemistry is a reliable and cost-effective screening method that supports MSI detection and guides genetic testing. Early diagnosis enables appropriate treatment and cascade screening of at-risk family members. ]]>
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