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


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](https://jisho.org/word/%E7%97%85%E6%B0%97), **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?**




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`](https://nihongokyoshi-net.com/2018/07/18/jlptn3-grammar-
wakedewanai/).  
This being said, `N+な` is the norm before `わけではない` so much so that `N+の` [can
feel incorrect](https://hinative.com/ja/questions/5421141).

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
>

* * *

There are also, more irregularly, in modern usage,
[cases](https://jn1et.com/noun-naadjective/) 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](https://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q1415952229)
discussing the acceptability of using `病気` as a 形容動詞.




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`](https://nihongokyoshi-net.com/2018/07/18/jlptn3-grammar-
wakedewanai/).  
This being said, `N+な` is the norm before `わけではない` so much so that `N+の` [can
feel incorrect](https://hinative.com/ja/questions/5421141).

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
>

* * *

There are also, more irregularly, in modern usage,
[cases](https://jn1et.com/noun-naadjective/) 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](https://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q1415952229)
discussing the acceptability of using `病気` as a 形容動詞.

