MCATin2018orBust

Hi all, i am prepping for the med school exam (MCAT) and I came across this molecule in my practice questions. I got the Q wrong and the explanations called this a Quaternary amine. I recognize that the nitrogen atom is Quaternary, but since only 3 of the 4 groups attached to the N are carbons (the last is sulfur), can I still count this as a Quaternary amine? Is tertiary amine with a Quaternary N more accurate? Thanks much.