You can't select anything when talking with the Guardians.
So it isn't possible to complete the Quest.
Link to Quest in wowhead (https://www.wowhead.com/quest=28034/lunks-adventure-rendans-weakness)
Quest in Searing Gorge
(Link to wowhead https://www.wowhead.com/quest=27958/a-proper-antivenom)
You need 14 Glassweb Venom from the Glassweb Spiders (NPC 5856), but you can't kill them. When the Health is near 50% it despawns for the Lurk Quest. (But without spawn Lurk!)