Can「病気【びょうき】」be used as a 形容動詞【けいようどうし】 (na-adjective)?

Question

I came across this multiple choice question as part of my homework. There is only one correct choice.

Question

心配【しんぱい】しないで。悪【わる】い病気【びょうき】 { ______ } と思【おも】います

Don't worry. I think { ______ } a bad illness/disease.

Answers

①　かもしれない it might be

②　なわけではない it's not that it's

③　だろう it probably is

At the first sentence, it is said "Do not worry", so it's clear that the disease should not be bad or serious. Therefore, the right answer is ②　なわけではない "it's not that it is [bad]". The other choices do not make sense because if the disease "might be [bad]" or "is probably [bad]", then you definitely should worry.

The problem is that choosing ② implies that 病気【びょうき】 is a 形容動詞【けいようどうし】 (na-adjective), because the choice contains a particle 「な」, rather than 「の」, but in my experience and according to Jisho.org, 病気 is not a 形容動詞【けいようどうし】 (na-adjective), but a 名詞【めいし】(noun).

②　なわけではない / のわけではない

心配【しんぱい】しないで。悪【わる】い病気なわけでわないと思【おも】います。

Is this a legitimate use of 病気【びょうき】, or maybe I picked the wrong answer in the first place?

Answer 1

First of all, you picked the right answer, and said right answer is also correct:

〇 悪い病気な訳ではない

As you reckoned, 病気 is usually not a 形容動詞 but a 名詞 only. It has more to do with how the idiom わけではない is used:
わけだ・ではない is, when modified by a noun, typically modified by the noun + な.

As you mentioned, when a noun (名詞) modifies (修飾する) another noun, it is typically appended with の and not な. However, for a reason I can't find a proper, grammatical explanation for, わけではない can be modified by (seemingly) both N+の or N+な.
This being said, N+な is the norm before わけではない so much so that N+の can feel incorrect.

As far as I can tell, this is peculiar to the idioms using わけだ・ではない. See for example other idioms with わけ :

• 悪い病気な訳だ
• 悪い病気の訳がない
• その言葉の訳が分からない

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There are also, more irregularly, in modern usage, cases where it can be deemed acceptable to make a na-adjectives out of what are traditionally only nouns (形容動詞化), but this is a different case and, I believe, not the reason for the validity of the N+なわけではない construct, as the acceptability of these is way more subjective / selective whereas N+なわけではない is valid with any noun. See for example this nice Q&A discussing the acceptability of using 病気 as a 形容動詞.