Okay, let's tackle this question. The patient is a middle-aged lady with lower back pain and weakness in the extension of the right great toe. No sensory issues. The question is asking about the likely level of a prolapsed intervertebral disc.

First, I need to recall the nerve roots involved in toe extension. The extension of the great toe is primarily controlled by the extensor hallucis longus muscle. The innervation of that muscle is from the deep peroneal nerve, which is a branch of the common peroneal nerve. The common peroneal nerve arises from the L4 and L5 spinal nerves. Wait, but the extensor hallucis longus is specifically innervated by the deep peroneal nerve, which is more L5 and S1. Let me confirm that. Hmm, maybe L5 more specifically. So, weakness in that movement would suggest an L5 nerve root compression.

Now, where is the L5 nerve root located? The spinal nerves exit below their corresponding vertebrae. So, the L5 nerve root exits between the L4 and L5 vertebrae and the L5-S1 disc. But wait, the intervertebral discs are between the vertebrae. So, the L5 nerve root exits below the L4-L5 disc. A prolapsed disc at L4-L5 would compress the L5 nerve root. Alternatively, a disc herniation at L5-S1 would compress the S1 nerve root. So, if the problem is with the L5 root, the disc herniation would be at L4-L5.

Let me check the possible options. The options aren't given, but the correct answer is likely L4-L5. The patient's symptoms point to L5 radiculopathy. Classic signs of L5 compression include weakness in dorsiflexion of the foot (due to peroneal nerve involvement) and great toe extension. Sensory changes might be in the dorsal foot, but the question says no sensory impairment, which is possible if the sensory fibers are less affected or the motor fibers are more prominently involved.

Wait, the extensor hallucis longus is L5, so if the L5 root is compressed, the disc herniation would be at the level where the L5 root exits. The L5 root exits between L4-L5 disc and the L5-S1 disc. Wait, actually, the L5 nerve root exits inferior to the L4-L5 disc. So a herniation at L4-L5 would compress the L5 root. Whereas a herniation at L5-S1 would compress the S1 root. Therefore, the correct answer is L4-L5 disc herniation causing L5 radiculopathy.

So, the MRI would show a prolapsed disc at L4-L5 level.

**Core Concept**
The question assesses knowledge of spinal nerve root innervation and intervertebral disc anatomy. The extensor hallucis longus muscle, responsible for great toe extension, is innervated by the **L5 nerve root**, which exits between the **L4-L5 disc** and the **L5-S1 disc**.

**Why the Correct Answer is Right**
A prolapsed disc at **L4-L5** compresses the **L5 nerve root**, leading to weakness in the extensor hallucis longus (EHL) muscle

✓ Correct Answer: B. L4-L5