Can I have a PhD in mathematics or physics after having a bachelor degree in medicine?
The hard answer is a straight no.

Doing research level Mathematics or Physics requires a broad set of knowledge and maturity that takes years to achieve. GRE exams are quite crappy, they just ask a lot of questions on basic knowledge and arithmetic; but you won't see an integration by parts (first year maths).

But not everything is lost. There are fields where medicine and maths overlap, and definitely your expertise on one field and interest in learning the other can definitely help. Look at Biomathematics, Bioinformatics, and perhaps, Biophysics. I myself am a Physicist working in Bioinformatics. I do pure analysis of the data, but in the group we also have a wet lab, where my colleagues are trying (among other things) to come up with new creative ways to gather data more suited for our purposes.