Urologic — Urinary retention

A 71-year-old man presents to the emergency department with 12 hours of worsening lower abdominal pain and inability to void. He has a long history of weak stream and nocturia, for which he has never been treated. Three days ago he began taking an over-the-counter cold and flu product containing pseudoephedrine and diphenhydramine. On examination there is a tender, distended suprapubic mass that is dull to percussion. Digital rectal examination shows a smooth, symmetrically enlarged prostate. There is no history of trauma, and no blood at the urethral meatus or perineal bruising. A bladder scan shows 850 mL. Creatinine is 168 µmol/L (60-110; baseline 92 µmol/L), potassium 4.8 mmol/L (3.5-5.0), and sodium 138 mmol/L (135-145). Ultrasound shows bilateral hydronephrosis.