## **Core Concept**
The patient's symptoms of sudden onset quadriparesis, positive Babinsky sign, increased muscle tone of limbs, and exaggerated tendon jerks suggest an upper motor neuron (UMN) lesion. The absence of sensory and sphincter involvement points towards a lesion in the cervical spinal cord. Rheumatoid arthritis (RA) can cause cervical spine instability, particularly atlantoaxial subluxation, which can compress the spinal cord.

## **Why the Correct Answer is Right**
The clinical presentation is suggestive of cervical cord compression, likely due to atlantoaxial subluxation in the context of RA. The best initial investigation to evaluate cervical spine instability and potential cord compression is an **X-ray of the cervical spine**, specifically looking for signs of atlantoaxial subluxation (e.g., increased atlanto-dental distance). This is a quick, readily available, and cost-effective initial imaging technique.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While MRI can provide detailed images of the spinal cord and surrounding structures, it is not typically the first-line imaging modality for assessing cervical spine instability due to its higher cost and limited availability compared to X-ray.
- **Option B:** CT scan can provide good detail of bony structures but is not the initial investigation of choice for assessing cervical spine instability in this context, especially when X-ray can quickly provide relevant information.
- **Option D:** Ultrasound is not typically used for evaluating cervical spine instability or spinal cord compression.

## **Clinical Pearl / High-Yield Fact**
In patients with rheumatoid arthritis, cervical spine instability, particularly atlantoaxial subluxation, is a serious complication that can lead to spinal cord compression. A high index of suspicion and early recognition are crucial. The atlanto-dental distance (the distance between the dens of C2 and the anterior arch of C1) is a key measurement; normally, it is less than 3 mm in adults.

## **Correct Answer:** . **X-ray**