Which of the following is considered a "red flag" symptom for low back pain?

Night pain is classified as a "red flag" symptom for low back pain because it may indicate the presence of serious underlying conditions such as malignancies or infections. Typically, pain that is worse at night and disrupts sleep can suggest that a more significant pathology is present, necessitating further investigation.

In contrast, morning stiffness can often be attributed to conditions like arthritis, which can sometimes be benign. Occasional pain with movement might indicate musculoskeletal strain but does not usually raise immediate concern for serious conditions. Mild discomfort with prolonged sitting is also generally not a specific indicator of serious illness and might be more related to lifestyle, posture, or minor musculoskeletal issues. Therefore, while all these symptoms warrant attention, night pain stands out as a symptom that may require urgent evaluation to rule out critical conditions.