The most serious immediate complication of acute urinary retention is bladder rupture. When the bladder becomes overly distended, the increased intravesical pressure can cause the bladder wall to tear, leading to urine extravasation into the peritoneal or extraperitoneal space. This results in an acute surgical emergency requiring prompt diagnosis and intervention to prevent peritonitis and sepsis. Other complications, such as urinary tract infection or hematuria, are important but generally less immediately life-threatening compared to bladder rupture. Early catheterization and decompression are essential to prevent this complication.

Reference: Campbell-Walsh Urology, 12th Edition.