Christina Applegate says she's back to "square one" after her MS misdiagnosis ... revealing she's getting tested for other diseases.

This week on her "MeSsy" podcast, the "Dead to Me" star revealed she's getting tested "one-by-one" for the "sister diseases" to MS.

She explained conditions like rheumatoid arthritis, lupus and Lyme disease can all mimic MS symptoms, saying ... "So now I’m being tested for all that stuff slowly but surely."

As you know, Christina revealed her MS diagnosis back in 2021 ... but announced just last month that she'd learned she was misdiagnosed.

The actress also mentioned on her podcast that she's been having a bit of an identity crisis in the wake of the misdiagnosis, and feels like "a fraud."

Hopefully Christina gets an accurate diagnosis soon.