De or het ritmestoornis?

The correct Dutch article is:

de ritmestoornis

Rule Words ending in -nis: usually de. See the rule

How to use ritmestoornis

the ritmestoornisde ritmestoornis
this ritmestoornisdeze ritmestoornis
that ritmestoornisdie ritmestoornis
our ritmestoornisonze ritmestoornis
your ritmestoornisjouw ritmestoornis
every ritmestoorniselke ritmestoornis
which ritmestoornis?welke ritmestoornis?
a big ritmestoornis / the big ritmestoorniseen grote ritmestoornis, de grote ritmestoornis

Dutch words ending in ritmestoornis

hartritmestoornis

In compound words, the last part decides the article. See the rule