Although you may think explaining that an author of one of your research paper's sources holds a professional degree like Doctor of Medicine, the American Psychological Association, or APA, style does not include indications such as M.D. in either in-text citations or the references list at the end.
In APA style, in-text citations include only the author's last name along with the date. First initials only appear if you have multiple authors with the same last name. Therefore, no professional degree indications appear.
A citation from a doctor would look like this: (Hazu, 2014). You may instead include information within a signal phrase to indicate the source's expertise, such as, "Dr. Hazu, an internist with 30 years' experience at the Mayo Clinic, disagrees."
Likewise, no indication of the doctor's professional degree appears on the references entry. The entry for Dr. Hazu would follow this structure:
Hazu, D. (2014). Pandemics in the 21st century. American Medical Journal, 13(3), 24-29.