A 33-year-old man comes to the clinic for 4 days of acute low back pain after lifting a heavy box at work. He describes a "tight, cramping" sensation in the paraspinal muscles and has difficulty sleeping due to intermittent muscle spasms. He has no fever, weight loss, lower-extremity weakness, saddle anesthesia, bowel/bladder dysfunction, or history of cancer. Examination shows localized paraspinal tenderness and spasm; straight-leg raise is negative bilaterally and neurologic exam is normal. He has already tried NSAIDs and heat with partial relief. The clinician prescribes an oral medication as a second-line option for acute nonspecific low back pain. Two days later, he reports significant dry mouth, constipation, and drowsiness. Which of the following medications was most likely prescribed?