**Question:** A patient of rheumatoid arthritis presents with progressive quadri-paresis and weakness, Babinsky sign was positive, increased muscle tone of limbs with exaggerated tendon jerks and worsening of gait with no sensory, sphincter involvement. Best initial investigation is-

A. Magnetic Resonance Imaging (MRI) of the spine
B. Electromyography (EMG) and Nerve Conduction Studies (NCS)
C. Serum Protein Electrophoresis (SPEP) and Bence-Jones Proteinuria
D. Serum Calcium and Parathyroid Hormone (PTH) levels

**Correct Answer:** B. Electromyography (EMG) and Nerve Conduction Studies (NCS)

**Core Concept:** Rheumatoid arthritis (RA) is a chronic inflammatory disorder that primarily affects the joints, leading to pain, swelling, and deformity. Involvement of other systems, such as the peripheral nervous system, is relatively rare, but can occur in severe cases.

**Why the Correct Answer is Right:** The correct answer, EMG and NCS, is chosen because these investigations are specifically designed to assess the function of the peripheral nerves and neuromuscular junction. In RA patients with neurological symptoms, these tests can help identify peripheral nerve involvement, such as demyelination or axonal degeneration, which may occur due to a direct infiltration of inflammatory cells into the nerves or by compression or infiltration of the nerves by synovial fluid.

**Why Each Wrong Option is Incorrect:**

A. MRI of the spine: Although MRI can show spinal cord compression in some cases, it is not specific for evaluating peripheral nervous system involvement in RA patients.

B. Serum Protein Electrophoresis (SPEP) and Bence-Jones Proteinuria: SPEP is used to detect abnormal proteins in the blood, particularly light chains (Bence-Jones proteins) that are indicative of plasma cell dyscrasias or multiple myeloma. These tests are not specific for evaluating peripheral nervous system involvement in RA patients.

C. Serum Calcium and Parathyroid Hormone (PTH) levels: Serum calcium and PTH levels are essential for the evaluation of hypercalcemia, hypocalcemia, or primary hyperparathyroidism, but they are not relevant to evaluate peripheral nervous system involvement in RA patients.

**Clinical Pearl:** In RA patients presenting with neurological symptoms, the initial workup should focus on peripheral nervous system involvement assessment. EMG and NCS are crucial tests for this purpose as they can identify peripheral nerve and neuromuscular junction pathology. Other tests like MRI, SPEP, serum calcium, and PTH levels are not specific for evaluating peripheral nervous system involvement in RA patients.