Spinal stenosis

A 35-year-old man comes to the clinic due to back pain. Upon entering the clinic, the patient is observed to be limping and appears in pain. He discloses back pain for the past few months that has been progressively worsening. Two days ago, he felt severe pain while lifting a box at home. The patient now describes the pain as dull, 7/10 in severity, which worsens with movement and radiates to the left thigh. He has noticed numbness in the perianal region while using the toilet. Vitals are within normal limits. On physical examination, focal tenderness is noticed over the lumbar and paraspinal area. There is a loss of sensation around the perianal area but a normal anal sphincter tone. Knee jerk reflex is normal but the ankle reflex is absent on the left. Strength is 5/5 on the right and 4/5 in the left gluteus maximus and hamstring muscles. Which of the following features will prompt the need for emergent imaging?