Abstract
This case report presents the diagnostic journey and management of a 19-year-old female who was diagnosed with advanced colorectal cancer (CRC) associated with Lynch syndrome (LS), a hereditary nonpolyposis colorectal cancer (HNPCC). The patient initially presented with complaints of persistent abdominal pain, vomiting, and unexplained weight loss, leading to investigations revealing hypochromic microcytic anemia and the presence of an inhomogeneous pelvic mass associated with the sigmoid colon. Subsequent diagnostic procedures, including flexible sigmoidoscopy and pathology reports, confirmed the presence of an advanced rectosigmoid adenocarcinoma with high-grade dysplasia. Molecular testing and immunohistochemical staining revealed a deficiency in mismatch repair proteins, confirming the diagnosis of LS. Despite ineligibility for certain clinical trials due to lymph node infiltration, the patient demonstrated a significant positive response to pembrolizumab immunotherapy, with a notable reduction in tumor size and lymph node involvement. This case underscores the significance of genetic predisposition in the development of early-onset CRC and the potential efficacy of immunotherapy in managing advanced CRC associated with LS in young patients. Additionally, this case provides insights into the evolving landscape of CRC management and the critical role of personalized treatment strategies in optimizing patient outcomes.