What would be your top 5 ddx for 29yo male with extreme fatigue, headaches, brain fog, impending doom/death (on occasion), orthostatic intolerance, post exertion malaise, and misc cardiovascular sx? all "standard" labs/imaging normal so far.

When all the labs: are normal it means that they haven't done the right test yet. It starts with a thorough history and physical exam! There will be a clue to your problems there. I would consider Lyme disease, depression, endocrine disorders, MS and toxic exposures. Good luck.

See below: #1 BY FAR would be Lyme disease with co-infections like Babesia, Bartonella & Mycoplasma. #2 other chronic infections such as EBV, HHV-6 and Chlamydia pneuoniae #3 Heavy metal toxicity (mercury, lead etc.) #4 Mold toxicity #5 Multiple food sensitivities related to parasites/dysbiosis It is possible you could have ALL of the above. "Standard" tests may miss all. See a good integrative doctor.