A week ago I noticed a few yellow blotches on some of my trichocereus bridgesii cactus which are approximately a year old now. A week later these yellow blotches seemed to get exponentially worse and I don't know what to do to treat them. I thought the soil might have been too dry for a long period of time so I watered the cactus but the problem is still there. Does anyone know what might be causing whatever this is and how I could go about treating it? I really don't want to loose something i've invested a lot of time into to go down the drain.

Could be mite damage. Try spraying with isopropanol (or a proprietary miticide if you do that kind of thing). I add essential oils like basil and cinnamon in the hope that they'll help kill off any undesirable critters. A bit of research will show you what further essential oils help best in the (cactus) garden.

Did you recently move them outdoors? It could be sunburn. If you are seeing damage near the tips, especially facing toward the south then sunburn could be the culprit. Its a good idea to gradually introduce them to full sun over the course of a couple of weeks when they are first brought outdoors in the spring.

Food grade diatomaceous earth works wonders for pest elimination as well. I like to imagine fungus gnats dying a slow and painful death.

I agree with Greyfox, with the top being the predominant area where the yellowing occurs is a tell that the sunlight may be a little too much for it. Although cactus are desert plants, remember that they start off very small, and usually grow in the shade under a larger plant. Therefore, if you do not allow the proper amount of shade on young and freshly transplanted cacti, you may get issues with sunburn. As long as you take care of it now there shouldn't be an issue in the future. That being said, I do not have the cactus here in front of me, so I can not weigh in a verdict with absolute certainty. I would still be on the lookout for any creepy crawlies in or around your soil and cactus, just to be safe. Hope you get to the bottom of it!

Thanks for help everyone. I don't think it's sunburn like some of you have suggested. The cactus have been slowly acclimated for ~ one year and when they do get sunburn they tend to be bruised purple in color, this is yellow. I'm afraid that the spidermites that infested the khat plants i've been growing nearby have traveled the distance and now infected the cacti (little flickers!). I'm not sure what to do now other then apply some neem oil since it's the only thing I have at hand atm.

If I had to take a guess I would say it is an infection (e.g. rust). Just a guess, I've never had that issue personally. Doesn't look like a sunburn to me though.

You can always check for mites. It's a lot easier with a hand lens (aka loupe, 16x works well for me).

KT - if you've had mites nearby then there's a high likelihood that's what it is. Even on the photo's there are some mite-sized specks visible when zoomed in.

Rust was my second guess, followed by (or in combination with) a nutrient deficiency. I'm thinking a spraying with horsetail (Equisetum) tea wouldn't hurt either.