De or het flebologie?

The correct Dutch article is:

de flebologie

Rule Words ending in -logie: always de. See the rule
Meaning (Dutch) (medisch) leer van de aderen en de ziekten daarvan

How to use flebologie

the flebologiede flebologie
this flebologiedeze flebologie
that flebologiedie flebologie
our flebologieonze flebologie
your flebologiejouw flebologie
every flebologieelke flebologie
which flebologie?welke flebologie?
a big flebologie / the big flebologieeen grote flebologie, de grote flebologie

Meaning, plural, diminutive, pronunciation and translations: WikiWoordenboek (Dutch Wiktionary) (CC BY-SA 4.0). Example sentences: Tatoeba (CC BY 2.0 FR).