Dr. Dayan
In Halacha, a physician usually brings expertise and details to a Posek regarding the nature of the halacha on the case. However, there are times when we trust the doctor's expertise and the matter of Halacha is clear, such as when a physician breaks Shabbos for a patient or permits a patient to eat on Yom Kippur. In some sense, a physician is like a Daayan, who rules in dinei nefashos (Tur, Y.D. 336).
Ein l’Dayan Ela Ma she’Einav Ro’os (Ruling on the facts based in front of him and what he believes to be correct) is an important concept when a Posek or Daayan rules on a case. Does the same apply to a doctor? For example, if two physicians disagree on whether a patient can fast on Yom Kippur, do we permit the patient to break their fast?
There is an opinion in the Gemara in Yoma (83a) that states that even if 100 doctors said it was okay for a sick patient to fast on Yom Kippur, and two physicians state that the patient should not fast, we follow the two doctors and rule that these patients must eat on Yom Kippur. The Shulchan Orech (O.C. 618) codifies this, although it is a machlokes in the Gemara and Rishonim. This ruling is based on the concept that the testimony of two people is equal to that of multiple people, including 100 or more. Additionally, since Pikuach Nefesh is always a significant factor in medical halacha, we are usually more lenient. Therefore, if a doctor thinks there is a risk to pikuach nefesh for a patient, that doctor's opinion must be heard and considered before a ruling is made.
Adapted from medicalhalacha.org.