It would be far better to have a letter who has had contact with you for a longer period than one week.
Furthermore, unless you've been employed full-time for a year or more, letters from classroom instructors and laboratory supervisors (course-work related or investigational) are what schools are looking for with regard to LORs as they are far more informative than letters from people who have known you in other capacities.
Unless the doctor is an American with some experience in medical school or residency admissions, you also run the risk of having someone who writes a letter that is culturally at odds with the customary LOR.