**Core Concept**
The patient's presentation of lower back pain exacerbated by walking or prolonged standing and occasionally relieved by bending over, combined with impaired sensation to light touch and vibration below the L4 dermatome, points towards a diagnosis of spinal stenosis. This condition is characterized by narrowing of the spinal canal, which can compress the spinal cord and/or nerve roots.

**Why the Correct Answer is Right**
Spinal stenosis is often caused by degenerative changes in the spine, including disc herniation, facet joint hypertrophy, and ligamentum flavum thickening. These changes can narrow the spinal canal, leading to compression of the spinal cord and/or nerve roots. In this patient, the impaired sensation to light touch and vibration below the L4 dermatome, along with absent deep tendon reflexes in the lower extremities, suggests compression of the L4 nerve roots. The fact that she occasionally experiences relief of symptoms with bending over may be due to temporary reduction in lumbar lordosis, which can increase the diameter of the spinal canal.

**Why Each Wrong Option is Incorrect**
**Option A:** Spondylolisthesis is a condition where a vertebra slips out of place, which can cause lower back pain and radiculopathy. However, it would not typically cause impaired sensation to light touch and vibration below the L4 dermatome, as the nerve roots would be compressed at a different level.

**Option B:** Herniated discs can cause lower back pain and radiculopathy, but they would not typically cause impaired sensation to light touch and vibration below the L4 dermatome without corresponding motor weakness.

**Option C:** Osteoporosis can cause vertebral compression fractures, which can lead to lower back pain. However, it would not typically cause impaired sensation to light touch and vibration below the L4 dermatome.

**Option D:** Scoliosis is a condition where the spine has an abnormal curvature. While it can cause lower back pain, it would not typically cause impaired sensation to light touch and vibration below the L4 dermatome.

**Clinical Pearl / High-Yield Fact**
In patients with spinal stenosis, walking or prolonged standing can exacerbate symptoms due to increased lumbar lordosis, which can narrow the spinal canal. Bending over or sitting can temporarily relieve symptoms by reducing lumbar lordosis and increasing the diameter of the spinal canal.

**Correct Answer:** C. Narrowing of the spinal canal with compression of the L4 nerve roots.