Acute Carecommon
A 46-year-old warehouse worker has had low back pain for three weeks since lifting a pallet. For the past two days the pain has run down both legs to the feet. This morning he could not feel the toilet seat, and he had to strain to start his stream and felt no urge to go beforehand. Pinprick is reduced over both buttocks and the inner thighs. Both ankle reflexes are absent. A bladder scan done immediately after he passed urine shows 420 mL. He is afebrile, has lost no weight and has no history of cancer. Which one of the following is the most appropriate next investigation?