secoooooo

Kind of makes you wonder, doesn't it? Maybe there was an overly generous CO who ended up getting fired for that, heh. But really, I've known so many people here who apart from having a stable (albeit low-paying) job, basically have no other ties to Russia and get approved. Weird.

Yup, as I said before, try seeing if there's any other kind of visa you might be eligible for. Student visa or some kind of visa that better fits your exact intention of why you want to go to the States, and therefore the officers wouldn't be skeptical about your exam/study intentions.
It's honestly not even about the income......it's more about the intention of the trip. In Russia, poor, low-income people get approved for tourist visas all the time (I know people personally who have), but they almost always have no interest in the US other than their trip. That's just how it is....find a visa whose intention matches your own, and you'll have much higher chance of approval. That's my advice.

I don't know about other consulates, but I've known people here who got tourist visas approved with extremely little financial evidence. Just a couple pay stubs from a few measly Russian salaries (usually around 400 bucks a month), marked "I will pay for my trip" and boom, approval. The key deciding factor it seems was that these people had zero ties to/interest in the US/US citizens other than just "I want to visit some touristy places!" Might just be a Russia thing, though. But I guess it was still their own bank account/job, so maybe that was the difference.

Professional exams can go both ways. A cousin of mine applied for a visitor visa several years ago for taking USMLE medical exams. Applied and got the visa in the first try. Took the step 1 exam, was grilled by CBP on the next entry to take step 2, but granted entry. Subsequently tried for a medical career in the US, didn't work out, so instead decided to be a doctor overseas.
No one taking a professional exam in the US ever keeps a US career off the table. The CO saw thru that in OP's case.

You can try to go to Canada instead.

Your reason makes sense but the type of visa you applied for casts some doubts.
I know that in other nations developing and underdeveloped nations, a degree or certificate from a US, Canada or UK is a huge boost for your career but that exams is not just for certificate. The main purpose is to practice in the US with the certification.
Your best bet is to apply to a school..Like for a master's program, then study for the exams and take it while you are doing your master's program and hopefully, you can switch over into a dental program or get an employer to sponsor you while you are here.
I remember in college, there were lots of Nigerian doctors studying for the foreign medical doctor exams of this kind. It wasn't easy. Only a few could pass the exams and move into medical practice in US. Majority ended up finishing the master's program they were enrolled in.
The visitor visa won't help you and you might never get it.
Depositing a lump sum amount into your bank account is a red flag. You need to show strong ties to your country that you will return when you apply for visitor's visa.
Student's visa would be better option.
good luck!

My guess is that since the exam is mainly for people who want to work/study in the US, they just simply don't trust that your ultimate goal is tourism/visitor.
I can see how a CO would interpret that as intent to immigrate. Adding US-required credentials to your foreign CV and then telling them it's merely for boosting your CV in your home country sort of doesn't add up to the (already skeptical) CO's. They are trained to reject everyone who seems like they aren't just going over for a quick vacation romp/temporary business conference....sometimes having "set plans" outside of tourism/conferences (like yours) hurts your chances even more.
I know it's lame, but this is just how it is. Plenty of people on this forum have had family members' tourist visas rejected (including me) and we just learn to deal with it. There often is little we can do to overcome the skepticism of the CO's.