kohen studying medicine

Rabbi Meir Orlian:

The "special bracelet" of Rav Goren is a metal item [specifically one of the six metals mentioned in the Torah. In practical terms – made of gold, silver, copper or iron (including steel)] that was defiled through contact with a dead body. The most common example is a ring, bracelet or necklace (made of one of these metals) that was brought to a funeral. His leniency is based on the combination of two factors.
First, a metal item that comes in contact with a dead body, has a level of defilement equivalent to that of a corpse. This law is known as cherev harei hu k'chalal, a sword [which contacted a corpse] is like the corpse. The generally accepted ruling is that even though the level of defilement is equivalent, the Torah prohibits a kohen to defile himself only to a corpse, but allows him to defile himself to such a metal item and wear it. (Rama Y.D. 369:1)
Second, many authorities maintain that a kohen who is touching a corpse does not incur an additional prohibition if he simultaneously contacts another corpse, since he is already in an equivalent state of defilement. (Rambam Hil. Avel 3:4)
Putting these two laws together, Rav Goren zt"l argued that a kohen who wears a metal item that was defiled (which is permissible), would not incur a prohibition when contacting a corpse, since it does not add to his level of defilement.
Other poskim, however, reject Rav Goren's comparison, since the metal band carries less severity than an actual corpse, even though its level of defilement is equivalent. Rav Goren himself acknowledges that this leniency is only in cases of great need. (His suggestion was made on a national level, in the context of allowing medical schools in Israel to accept kohanim.)
Also see in Assia Book 15 [Hebrew], the Schlesinger Institute 5776, section III – Cohanim and Medicine, pp. 157-273.