I am almost certain that you cannot. You need the letter to be from a science faculty at your school, aka professor. Although this may differ from school to school. 👍
most schools no. some very rare schools will not have any rules about where your lors are coming from, but for the most part, no. you may have it in as an additional character letter but not a replacement for a mandatory teaching faculty letter
well my 4th letter is from my internship at my schools sports medicine program. Technically I was given a grade on my transcript for 2 years which was labeled under research units. My adviser says that might work because i do not have another quality science professor to right a letter.
i had an MD write my science LOR. But I took a course in neuroanatomy with him. Bottom line, if the MD taught you a course in which you got a grade, you'll be fine. an MD you only shadowed will not count