De or het melkallergie?

The correct Dutch article is:

de melkallergie

How to use melkallergie

the melkallergiede melkallergie
this melkallergiedeze melkallergie
that melkallergiedie melkallergie
our melkallergieonze melkallergie
your melkallergiejouw melkallergie
every melkallergieelke melkallergie
which melkallergie?welke melkallergie?
a big melkallergie / the big melkallergieeen grote melkallergie, de grote melkallergie

Dutch words ending in melkallergie

koemelkallergie

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