Instead of just "The term 'MedicalEntity' is not in the schema.org core vocabulary, but is described by the following extension(s):" let's add:

"Note: extension terms can be used in schema.org markup in the normal manner; it is not necessary for markup publishers to indicate which extension (currently) a term is in."

The options around JSON-LD are complex and evolving but I think we can safely say this immediately, to reassure some publishers. See also: #1164