It's far easier to forget data from memorization-based subjects. My resident friends have already forgotten a lot from the 1st two years of medical school, especially first year. I don't think anyone of them remembers much about the pterygopalatine fossa which I'm memorizing now. It's much easier to retain info from principle-based subjects such as chemistry, physics and mathematics. I still remember a fair amount of organic chemistry: Markovnikov regiochemistry, Diels-Alder, Grignard reagents/reaction for R substitutions, resonance theory, some principles applying to reactions of the functional groups, carbocations, ozonolysis, ring strain and reactivity, free energy from physical chem, force fields, newtonian mechanics, pressure/bernoulli, waves, heat, kinetic theory, etc. I don't remember how to solve a second-order ODE by hand, but I still remember the principles. I may not know how solve problems involving constraint-based max and mins using Lagrange multipliers but the general principle remains with me.
Usually if you don't use some data you'll forget them. I remember repeatedly memorizing amino acids and their side chains in college because I was forgetting. However my PhD thesis required extensive cloning, mutational analysis and biophysics such that the structures of amino acids just stayed with me. I can still easily draw nearly all of them now. I also knew most of the genetic code at one point due to carrying out lots of site-directed mutagenesis/cloning. I still remember a few, but I'm already forgetting.
