A Clinical Trial of Human Umbilical Cord Mesenchymal Stem Cell Injection for the Treatment of Severe Acute Respiratory Distress Syndrome

Inclusion Criteria: * Male or female patients aged 18 to 80 years (inclusive). * Diagnosis of moderate or severe Acute Respiratory Distress Syndrome (ARDS) according to A New Global Definition of Acute Respiratory Distress Syndrome, with an infectious etiology. * No improvement after 24 hours of conventional clinical treatment (defined as a persistent PaO₂/FiO₂ ratio ≤200 mmHg or a decrease from \>200 mmHg to ≤200 mmHg after 24 hours of conventional supportive therapy; for severe ARDS, this assessment period may be shortened to 8 hours). * Ability to fully understand the nature of the study and voluntarily provide written informed consent. * Willingness to comply with all study procedures and demonstrate good compliance during the study period. * Agreement to participate in long-term follow-up. Exclusion Criteria: * Patients with ARDS caused by COVID-19 infection. * Patients currently suffering from hepatitis B, hepatitis C, active or latent tuberculosis, AIDS, syphilis, immunodeficiency disorders, or other immune system diseases. * Presence of severe cardiovascular diseases at screening, including:Cardiac function classification of NYHA class III or higher.Uncontrolled myocarditis or valvular disease.Malignant arrhythmia requiring pharmacological treatment. * Abnormal liver or renal function at screening meeting any of the following criteria:ALT or AST ≥ 5 × ULN, or total bilirubin ≥ 3 × ULN.Serum creatinine ≥ 3 × ULN, or patients currently undergoing renal replacement th…