1. Key Clues:

- "Rare instances" $\(\rightarrow\)$ The condition is uncommon.
- "Congenital deficiency" $\(\rightarrow\)$ The blank should reflect abnormality or rarity.

2. Evaluate Options:

(A) atypical (unusual) $\(\rightarrow\)$ Fits; directly aligns with "rare."
(B) phenomenal (remarkable) $\(\rightarrow\)$ Incorrect; implies impressiveness, not rarity.
(C) unparalleled (unique) $\(\rightarrow\)$ Overstated; rarity doesn't imply singularity.
(D) peculiar (uncommon/distinctive) $\(\rightarrow\)$ Fits; synonymous with "atypical" here.
(E) eccentric (odd behavior) $\(\rightarrow\)$ Incorrect; describes personality, not medical conditions.
(F) arcane (obscure) $\(\rightarrow\)$ Incorrect; implies secrecy, not rarity.

Correct Answers: (A) atypical and (D) peculiar.

It is a matter of nuances, sometimes