A 45-year-old man is unable to void 8 hours after haemorrhoidectomy under spinal anaesthesia with bupivacaine. The primary mechanism for this postoperative urinary retention is:

• A Sacral micturition center blockade ✓
• B Bladder neck obstruction
• C Detrusor muscle overactivity
• D Urethral edema

Explanation

Spinal anaesthesia blocks the sacral parasympathetic outflow (S2-S4), inhibiting detrusor contraction and the micturition reflex. This blockade persists until the sensory level recedes below S2. Opioids and anticholinergics worsen retention. Detrusor overactivity would cause incontinence, not retention. Bladder neck obstruction and urethral edema are not the primary mechanisms in this setting.

Reference: Barash's Clinical Anesthesia, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.