Professional and Linguistic Assessments Board (PLAB) Practice Exam

What characterizes the myelin layer in subacute combined degeneration of the cord?

1. Asymmetric degeneration

2. Inflammation of the myelin

3. Symmetric myelin layer vacuolization

4. Malformation of axons

The correct answer is: Symmetric myelin layer vacuolization

The myelin layer in subacute combined degeneration of the cord is characterized by symmetric myelin layer vacuolization. Subacute combined degeneration is primarily associated with vitamin B12 deficiency and affects both the dorsal columns and corticospinal tracts of the spinal cord. In this condition, the myelin sheath surrounding the axons becomes compromised, leading to the formation of vacuoles within the myelin. This vacuolation occurs symmetrically, reflecting the bilateral nature of the pathology in the affected tracts. The symmetric aspect is significant as it helps to differentiate this condition from others where degeneration may be more asymmetric, affecting only one side of the cord. Additionally, symmetric vacuolization corresponds with the overall pathology observed in vitamin B12 deficiency, which leads to demyelination and subsequent neurological symptoms. Understanding these characteristics is crucial for recognizing subacute combined degeneration in clinical settings, which may guide appropriate management and treatment strategies.