A 58-year-old Indian male presents with progressive difficulty walking, positive Romberg sign, and loss of vibration sense in both lower limbs. Serum B12 level is 180 pg/mL (normal 200). MRI spine shows T2 hyperintensity in the dorsal midline of the cervical and thoracic cord. The structure marked **B** in the diagram is primarily affected in this condition. Which of the following best explains the clinical presentation?

Question

A 58-year-old Indian male presents with progressive difficulty walking, positive Romberg sign, and loss of vibration sense in both lower limbs. Serum B12 level is 180 pg/mL (normal >200). MRI spine shows T2 hyperintensity in the dorsal midline of the cervical and thoracic cord. The structure marked **B** in the diagram is primarily affected in this condition. Which of the following best explains the clinical presentation?

Accepted Answer

A. Demyelination of ipsilateral dorsal columns carrying proprioception and vibration sense, with fibers ascending uncrossed to the medulla before synapsing in dorsal column nuclei. ## Why option 1 is right

Subacute combined degeneration (SCD) from vitamin B12 deficiency classically affects the dorsal columns (structure **B**) and lateral corticospinal tracts. The dorsal columns carry ipsilateral fine touch, vibration, and proprioception as uncrossed fibers that ascend to the medulla, where they synapse in the gracile and cuneate nuclei before decussating as internal arcuate fibers to form the medial lemniscus. Demyelination of the dorsal columns explains the loss of vibration sense and positive Romberg sign (proprioceptive loss). The patient's clinical triad of sensory ataxia, positive Romberg, and loss of vibration/proprioception is pathognomonic for dorsal column involvement. (Gray's Anatomy 42e Ch 23; Harrison 21e Ch 437)

## Why each distractor is wrong

- **Option 2**: While lateral corticospinal tract involvement does occur in SCD and contributes to spastic paresis and hyperreflexia, it does NOT explain the loss of vibration sense and positive Romberg sign. These findings are specific to dorsal column pathology, not pyramidal tract disease.
- **Option 3**: The spinothalamic tract (structure **C**) carries pain and temperature sensation in a crossed fashion. Spinothalamic lesions cause dissociated sensory loss with preservation of vibration and proprioception—the opposite of this patient's presentation.
- **Option 4**: Anterior horn cell degeneration (structure **D**) causes lower motor neuron weakness with fasciculations and atrophy. This patient has upper motor neuron signs (hyperreflexia, spasticity) and pure sensory loss, not motor neuron disease.

**High-Yield:** Dorsal columns = ipsilateral uncrossed vibration + proprioception; SCD = dorsal columns + lateral corticospinal tracts = sensory ataxia + spastic paresis + Romberg positive.

[cite:Gray's Anatomy 42e Ch 23; Harrison 21e Ch 437]

Answer

B. Demyelination of ipsilateral lateral corticospinal tract causing loss of motor control and hyperreflexia in lower limbs

Answer

C. Demyelination of ipsilateral spinothalamic tract causing dissociated sensory loss (pain and temperature preserved)

Answer

D. Degeneration of ipsilateral anterior horn cells causing lower motor neuron weakness with fasciculations

Explanation

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