The funniest thing is I can’t say what I complain of.

[englishhobby]

Is this senternce grammatical? The context is as follows: a patient comes to the doctor's and can't explain what is wrong with him.

 

[emsr2d2]

I would use "The funny thing is that I can't really explain to you what's wrong".

Even though it is the doctor who will determine what is actually wrong, we tend to use the phrase ourselves to mean that we don't feel "right" (ie completely healthy).