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Because in vitamin B12 deficiency specifically, the condition known as subacute combined degeneration (SCD) affects both the dorsal columns and the corticospinal tract. A lesion in the dorsal columns will lead to loss of position sense, which is crucial to such tests as knee and ankle jerk tests. So, these tests will show areflexia. Additionally, a lesion in the corticospinal tract will lead to a positive Babiniski sign. It will not be able to induce hyperreflexia because the afferent of the reflex arc itself is affected by the lesion in the dorsal columns.
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