The most useful investigation for determining the cause of acute urinary retention is digital rectal examination (DRE). This allows direct assessment of the prostate gland to identify enlargement, nodules, or tenderness, which are common causes of obstruction in males. It also helps evaluate for rectal masses or neurological deficits affecting the pelvic floor. Unlike urinalysis or imaging, DRE provides immediate clinical information to guide management. This examination is essential in differentiating obstructive causes from infection or other pathologies.

Reference: Campbell-Walsh Urology, 12th Edition.