Brandi Glanville's mystery illness has gotten so serious to the point she needs to go under the knife ... and Dr. Terry Dubrow is doing everything he can to get her face back to the way she wants it.

Dr. Dubrow tells TMZ ... Brandi is continuing to experience uncomfortable symptoms from her mystery illness, and will require a series of tests to get an exact diagnosis. To get to the bottom of it, Dubrow will perform a surgical tissue sampling, which will hopefully provide clarity to the root cause of the issue.

Brandi is feeling somewhat better after being recommended treatment from Terry last month. The good doc commends Brandi for her bravery during this process and is determined to figure it out.

Glanville tells TMZ ... working with Dubrow is a no-brainer, considering he deals with medical mysteries for a living. With that, Brandi says she's willing to work with anyone qualified and able to provide a definitive diagnosis.

It's surprising Brandi isn't back up to speed ... a few days ago, she posted photos of herself in a pink bikini, and looks fantastic.

As you know, Glanville has met with a variety of medical professionals to help with her face disfiguration. BG has been transparent on the toll it's taken, saying she hasn't had sex since October 2023.

Regardless of her condition, Brandi says she feels very blessed with all the support she's receiving from people offering to help.