Well to start with, there are very few MD schools that need/want a clinical letter. So don’t pick your job based on a letter. (DO schools usually do require a letter). I’d pick a job where you work primarily with a physician , either MD or DO. You are applying to medical school, not PA school. So work with a doctor. I know that sometime will probably be spent with PAs, nurses, APNs, etc but the majority of your experience should be focused on doctors. Of your list of opportunities I’d probably do rheumatology or family medicine. But it’s up to you.
