A 60-year-old patient with low back pain radiating down the outer thigh and leg most likely has which pathology?

• A. Microtears of the ligamentum flavum and posterior longitudinal ligament
• B. L3-L4 lumbar facet joint arthropathy
• C. Soft tissue strain of the lumbar extensors
• D. Posterolateral protrusion of the lumbar intervertebral disk at L5-S1

Answer: (D)

When pain travels down the leg in a dermatomal pattern, think nerve root compression with radiculopathy rather than just local back strain. The most common cause in this scenario is a posterolateral disk herniation in the lumbar region. Material herniating laterally/posterolaterally is able to press on the traversing nerve root within the foramen, especially at the L5–S1 level.

At L5–S1, a posterolateral herniation typically compresses the S1 nerve root. The S1 distribution runs along the posterior and lateral aspects of the leg into the foot, so pain that radiates down the outer thigh and leg fits with radicular pain from an S1 involvement. You might also see signs like weakness with ankle plantar flexion and a diminished Achilles reflex, though not always present.

The other options describe structures or conditions that usually cause localized back pain or referred pain to the thigh or buttock without the clear radicular pattern down the leg. Therefore, the posterolateral lumbar disk protrusion at L5–S1 best explains the radiating leg pain.