## **Core Concept**
Subacute combined degeneration (SCD) is a condition caused by vitamin B12 deficiency, primarily affecting the spinal cord. It involves both the posterior columns and the corticospinal tracts, leading to a combination of sensory and motor symptoms.

## **Why the Correct Answer is Right**
The correct answer involves the posterior column tract (c) and corticospinal tract (b), which are the primary sites of pathology in subacute combined degeneration. Vitamin B12 plays a crucial role in the synthesis of myelin and the maintenance of the nervous system. Its deficiency leads to demyelination in these tracts, resulting in impaired proprioception (posterior column) and motor function (corticospinal tract).

## **Why Each Wrong Option is Incorrect**
- **Option A:** Peripheral nerve - While vitamin B12 deficiency can cause peripheral neuropathy, subacute combined degeneration primarily involves the spinal cord, not the peripheral nerves.
- **Option D:** Spinothalamic tract - The spinothalamic tract, responsible for transmitting pain and temperature sensation, is not the primary site of involvement in SCD.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that patients with subacute combined degeneration often present with a triad of symptoms: weakness (due to corticospinal tract involvement), loss of vibration and proprioception (due to posterior column involvement), and psychiatric manifestations. Early recognition and treatment with vitamin B12 can significantly improve outcomes.

## **Correct Answer:** C. posterior column tract.