PD-1 + FOLFOXIRI vs CAPOX as Total Neoadjuvant Therapy for pMMR Low Rectal Cancer

Inclusion Criteria: * Histologically confirmed rectal adenocarcinoma. * pMMR status documented by immunohistochemistry (MSH1, MSH2, MSH6 and PMS2 all positive) or by MSI testing showing MSS or MSI-L. * Primary tumor located within 5 cm from the anal verge on pelvic MRI. * Clinical stage cT3-4bN0M0 or cTxN+M0, with or without positive mesorectal fascia (MRF) and/or extramural vascular invasion (EMVI), and assessed by the multidisciplinary team as resectable with the potential for R0 resection. * No clinical or radiologic evidence of bowel obstruction. * No prior colorectal surgery. * No prior chemotherapy or radiotherapy. * No prior treatment with biologic agents (e.g., monoclonal antibodies), immune checkpoint inhibitors (e.g., anti-PD-1, anti-PD-L1, anti-PD-L2, anti-CTLA-4), or other investigational drugs. * Age 18 to 75 years (inclusive). * Eastern Cooperative Oncology Group (ECOG) performance status 0-1. * Estimated life expectancy \> 2 years. * Adequate bone marrow function: WBC \> 3.5 × 10\^9/L; platelets \> 100 × 10\^9/L; hemoglobin \> 80 g/L. * Adequate liver function: ALT and AST ≤ 1.5 × upper limit of normal (ULN); total bilirubin \< 23.0 μmol/L. * Adequate renal function: eGFR ≥ 60 mL/min/1.73 m² (CKD-EPI) or creatinine clearance (Ccr) ≥ 60 mL/min. * Ability to understand and willingness to sign a written informed consent form. Exclusion Criteria: * Evidence of inguinal or lateral pelvic lymph node metastasis (lymph node short-axis diameter ≥ 7 mm or MRI features typical for metastatic lymph nodes). * Clinically significant cardiac disease, including arrhythmias requiring anti-arrhythmic treatment (except β-blockers or digoxin), symptomatic coronary artery disease or myocardial ischemia (myocardial infarction within the last 6 months), or congestive heart failure \> NYHA class II. * Uncontrolled severe hypertension. * History of HIV infection or active chronic hepatitis B (HBV DNA \> 2 × 10\^3 IU/mL) or hepatitis C (HCV RNA \> 1 × 10\^3 IU/mL). * Active pulmonary tuberculosis under treatment or having received anti-tuberculosis therapy within 1 year before screening. * Other active severe infections (as defined by NCI-CTCAE v5.0). * Evidence of distant metastasis outside the pelvis before treatment. * Cachexia or decompensated organ dysfunction. * Prior pelvic or abdominal radiotherapy. * Multiple primary colorectal cancers. * History of seizures requiring ongoing treatment (e.g., corticosteroids or anti-epileptic drugs). * History of another malignancy within the past 5 years, except adequately treated cervical carcinoma in situ or basal cell carcinoma of the skin. * Drug or alcohol abuse, or any medical, psychological, or social condition that, in the investigator's judgment, could interfere with study participation or the evaluation of study results. * Any active autoimmune disease or history of autoimmune disease (including but not limited to interstitial pneumonia, uveitis, colitis, hepatitis, hypophysitis, nephritis, hyperthyroidism, or hypothyroidism). * Receipt of any prophylactic vaccine against infectious diseases (e.g., influenza, varicella) within 4 weeks before enrollment. * Requirement for long-term immunosuppressive therapy or systemic/local corticosteroids at immunosuppressive doses (equivalent to \> 10 mg/day of prednisone or other corticosteroids). * Known or suspected allergy or hypersensitivity to any study drug or to any component of the study treatments. * Any unstable condition or other circumstance that may, in the opinion of the investigator, compromise patient safety or study compliance. * Pregnant or breastfeeding women, or women of childbearing potential who are unwilling or unable to use adequate contraception during the study. * Refusal or inability to provide written informed consent.