This quest blends with the quest "The Collective Base", as it begins when the former ends. As you talk to Crux to end the aforementioned quest she’ll mention that somebody has been impersonating the Charlatan, to the detriment of other Collective operatives. Of course, an internal parasite knows the chain of command, and will be able to avoid an investigation better, but an outside might just be able to glean more information off of a guilty party… especially one armed with a magical AI-power scanner.
Offer to help Crux investigate, and note the two leads you have; the footprints of the perpetrator belong to either an Asari or a human (convenient how beings so similar evolved entirely independently, untold light years away!) and given the scene of the last crime they should have higher-than-usual amounts of sulfur on their clothes. Not like they have a chance to wash it off, either, given water is a precious commodity on Kadara, and the native water is likely to take your skin off with it.
With that in mind, turn your attention to the five targets to scan. There’s one in the main floor of the collective base (a Salarian - ruled out), another on the walkway outside (no sulfur - ruled out), an Angara cultivating some poisonous mushrooms east of the base (again, wrong footprints - ruled out). Three strikes! Below the base you’ll find the Asari who greeted you at the door - Lynx - who both has the right hooves and a lot of sulfur on her clothes. Finally, near the Adhi kennels northwest of the base you’ll find a human female named Dorado, who also shares the two diagnostic traits, and is nervous, to boot.
Now that your suspects are narrowed down, talk to them. Dorado is rather rude (seeming kind of guilty with that attitude!) and on the second dialogue choice you’ll get two innocuous options: "You’re completely loyal, huh?" and "Like the sulfur springs.". Remember that you’re supposed to keep a low profile, and in this case, the option that seems less accusatory is actually the wrong one. If you say "You’re completely loyal, huh?" Ryder will - like a world-class dingus - question Dorado’s loyalty, spooking her. On the other hand, outright accusing her of being at the scene of the crime (picking the dialogue option "Like the sulfur springs." is the the correct choice if you want to keep a low profile.
Once Dorado’s been chatted up SAM and Ryder will chat. Before bothering Crux, however, you still have to chat with Lynx, to return to her and again you’ll get two dialogue options (the second choice you make is the one that matters). If you say "Explain the sulfur" Lynx will rightly take offense at your creepy scrutiny, while if you say "Tough break" she’ll mention working on the water filters, which is a fine alibi for her sulfur-stank.
After talking to both Dorado and Lynx (and ideally keeping a low profile both times) return to Crux and finger Dorado. If you said "Like the sulfur springs." you’ll have done your job properly and Dorado will be apprehended with no fuss. On the other hand, if you said "You’re completely loyal, huh?" you’ll have alerted the flighty Dorado, who will have run off after you left. To fix your mistake Ryder will offer to hunt Dorado down, forcing you to backtrack into the caverns, where Dorado will ambush you near where you fought the first Rylkor. Put her down and after a comm chat with Crux you’ll have made good on your promise. Either way, once Dorado is dealt with you’ll earn 29 AVP, +2% Kadara viability and 530 XP.