I started the Venom Bottles quest in The Hinterlands and went to complete in Tarren Mill, as per the quest. You're supposed to give the Venom Bottle quest item to Apothecary Lydon, but he doesn't have the quest completion question mark on him. You physically cannot complete the quest because of this. I've checked the vanilla wiki and everything should be working, I've tried relogging and restarting the game but it still doesn't work. Below is a link to a screenshot showing that I have the quest active, have the quest item in my inventory, and the NPC who doesn't have the yellow question mark.

http://prntscr.com/o1tz2w