GMPA Versus PPI for Gastric Protection and aGVHD Risk

Inclusion Criteria: 1. Age 12-70 years; 2. First allogeneic haploidentical HSCT for hematological malignancy; 3. Treated with a standardized myeloablative conditioning and GVHD prophylaxis regimen; 4. ECOG performance status ≤2 prior to transplant; 5. Written informed consent obtained. Exclusion Criteria: 1. Pre-existing conditions requiring continuous PPI therapy prior to transplant, including severe reflux esophagitis, Zollinger-Ellison syndrome, active peptic ulcer with bleeding, or long-term use of dual antiplatelet agents or oral anticoagulants; 2. Known active, untreated Helicobacter pylori infection prior to transplant; 3. Active infectious enteritis or Clostridioides difficile infection prior to transplant; 4. Use of systemic broad-spectrum antibiotics within 7 days prior to transplant; 5. Use of probiotics within 14 days prior to transplant; 6. Planned total enteral nutrition or oral glutamine supplementation during the study period; 7. Planned parenteral nutrition for ≥7 consecutive days; 8. Inability to provide saliva or stool samples for collection; 9. Pregnant or lactating women; 10. History of significant neurological or psychiatric disorders (e.g., epilepsy, dementia) that may interfere with study participation; 11. Life expectancy \<3 months or severe end-organ dysfunction; 12. Any other condition that, in the investigator's judgment, may compromise patient safety, compliance, or the validity of study results.