**Core Concept**
The patient's symptoms are suggestive of a lumbar disc herniation, which is a common cause of lower backache. The pain is exacerbated by bending down, which increases the pressure on the intervertebral disc.

**Why the Correct Answer is Right**
The patient's symptoms are consistent with a lumbar disc herniation, which occurs when the soft, gel-like center of the disc bulges out through a tear in the outer, fibrous ring. This can irritate the nearby nerve roots, leading to pain, numbness, and tingling in the lower back and legs. The pain is often worsened by bending, lifting, or twisting, as these movements increase the pressure on the disc.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect, as a lumbar disc herniation typically presents with pain in the lower back and legs, not just the lower back.
**Option B:** This option is incorrect, as osteoporosis typically presents with pain in the spine, but it is often accompanied by other symptoms such as loss of height, kyphosis, and decreased bone density.
**Option C:** This option is incorrect, as a lumbar strain typically presents with pain and stiffness in the lower back, but it is often accompanied by muscle spasm and decreased range of motion.
**Option D:** This option is incorrect, as a kidney stone typically presents with severe, colicky pain in the flank or abdomen, often accompanied by hematuria and nausea.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that a lumbar disc herniation can be a "red flag" symptom, indicating a potential neurological emergency. If the patient experiences numbness, tingling, or weakness in the legs, or if the pain is severe and worsening, prompt evaluation by a neurosurgeon or orthopedic surgeon is necessary.

**Correct Answer:** C.