Blood at the urethral meatus is the most reliable clinical sign indicating urethral injury because it directly reflects trauma to the urethra causing bleeding. This sign strongly suggests disruption of the urethral mucosa and warrants further investigation before catheterization to avoid worsening the injury. While other signs like perineal bruising, inability to void, or a high-riding prostate may be present, they are less specific or may be absent early on. Identifying blood at the meatus prompts immediate retrograde urethrography to confirm diagnosis and guide surgical management. Avoiding blind catheter insertion is crucial to prevent conversion of a partial tear into a complete disruption.

Reference: Campbell-Walsh Urology, 12th Edition.