**Core Concept**
The patient's symptoms of lower back pain and hypoesthesia in the left S1 dermatomal distribution suggest a lesion affecting the S1 nerve root, which is a part of the lumbosacral plexus. The S1 nerve root is responsible for innervating the calf and lateral foot.

**Why the Correct Answer is Right**
The most likely cause of this presentation is a herniated lumbar disc, specifically at the L5-S1 level, which is a common location for disc herniation. The herniated disc compresses or irritates the L5 nerve root, which then compresses or irritates the S1 nerve root, leading to the patient's symptoms. The L5-S1 disc is more prone to herniation due to its anatomical location and the high mobility of the lumbosacral joint.

**Why Each Wrong Option is Incorrect**
**Option A:** A spinal cord lesion would more likely cause bilateral symptoms due to the bilateral innervation of the spinal cord.
**Option B:** A peripheral nerve injury would likely cause more diffuse or patchy sensory loss, rather than a well-defined dermatomal distribution.
**Option C:** A musculoskeletal cause, such as a muscle strain, would not typically cause sensory loss.

**Clinical Pearl / High-Yield Fact**
In patients presenting with lower back pain and radiculopathy, it's essential to perform a thorough neurological examination, including a dermatomal examination, to identify the affected nerve root and guide further investigations.

**Correct Answer:** C.