I don't know what step I does test, but it sure as hell doesn't test "general medical knowledge."
You could easily be an excellent doctor and fail step I.
And for anyone who disagrees with that, I went to a CME class at a medical conference by accident, and they were covering basic, basic molecular biology like the role of cytokines in metabolic syndrome... and literally everyone in the room looked like they were being told Santa was real. This isn't sour grapes, I did pretty well. But I'm not going to look back in fondness, in terms of future diagnostic utility and by any other metric than self-flagellation, step I studying was a giant waste of time.
I'm still waiting for that fantasy scenario where I get paged at 2 AM and the nurse is like
WITHOUT ANY OTHER DIAGNOSTIC INFORMATION OR EVEN GENERAL DEMOGRAPHICS REGARDING THE PATIENT, DOCTOR, WHAT IS THE CARDIAC PATHOPHYSIOLOGY INVOLVED IN AN ISOLATED INCIDENCE OF ELEVATED URINARY POTASSIUM. YOU HAVE 70 SECONDS OR THEY DIE
To answer your question: yes, climbing up the tail of a bell curve is by definition going to be exponentially more difficult the further you go.