Which finding in a postoperative patient suggests an urgent need to apply oxygen therapy?

Hypoxemia is reduced arterial oxygen saturation with SpO2 <90%, impairing oxygen delivery, causing cellular hypoxia, lactic acidosis, and organ dysfunction. Postoperative patients are predisposed due to anesthesia effects, hypoventilation, atelectasis, and ventilation-perfusion mismatch. Rationale: A. Requesting additional pain medication indicates inadequate analgesia rather than impaired oxygenation. Pain may increase sympathetic activity but does not directly signify hypoxemia. This finding does not demonstrate compromised oxygen saturation or impaired respiratory gas exchange requiring oxygen therapy. B. SpO2 level dropping below 90% indicates significant hypoxemia and inadequate arterial oxygenation. Immediate oxygen therapy is required to restore adequate tissue oxygen delivery and prevent complications. This reflects impaired gas exchange and risk of respiratory and cardiac deterioration. C. Mild discomfort at the surgical site is an expected postoperative finding related to tissue trauma. It does not indicate respiratory compromise or oxygenation issues. This symptom does not affect oxygenation status or necessitate immediate oxygen therapy intervention. D. Slight increase in heart rate after walking is a normal physiologic response to activity. It reflects increased metabolic demand rather than hypoxia. This does not indicate impaired respiratory function or the need for supplemental oxygen.