In Portuguese, it is common to say that a patient is "undergoing an investigation" of their disease (meaning: the physician is trying to diagnose what they have, i.e., is "investigating" their condition).
This, however, seems wrong in English, I think "investigation" has a sort of legal connotation usually. I'm also unsure whether it's grammatical to say "patient X is undergoing an investigation of / investigating their disease".
From what I gather, "clinical investigation" tends to mean the same as "clinical trials", a research trying to find new treatments or discover new diseases. This isn't the meaning I'm looking for; rather than a case study of sorts, I'd like to refer only to the struggle of a patient and their doctor in finding out which disease they have.
What is an idiomatic and formal way to say that my patient X is "trying to find out" what disease they have?