FA 14 and FA 15 discrepancy (Subacute Combined Degen)

B12 deficiency causes the classic SCD (spinocerebellar, corticospinal, dorsal columns).

Vitamin E deficiency can lead to hemolysis and definitely neurological dysfunction as well. It was removed from FA because of the overwhelming number of errata submissions and the confusion it was causing. And the picture often isn't the classic SCD. That term SCD is essentially synonymous with neuro dysfunction secondary to B12 deficiency if it were ever mentioned in a medical setting.

On the USMLE:

If you get neuro dysfunction in any capacity and no other helpful information (and it's not an INH-B6 question), the answer is B12 deficiency.

If you get neuro dysfunction and hemolysis, the answer is vitamin E deficiency.

If you get neuro dysfunction and they say anything about high serum methylmalonic acid/methylmalonyl-CoA, the answer is B12 deficiency.

And of course any mention of increased MCV and/or hypersegmented neutrophils with neuro dysfunction = B12 deficiency.

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FA 14 and FA 15 discrepancy (Subacute Combined Degen)