In spanish you also have to use a/an:

Es un perro (its a dog)

Saying "es perro" makes no sense.

The problem is that you are mixing two different constructs, if you are refering to the whole thing you have to say un/una, if you are describing a property of something you don't use it.

Es un perro, es macho. (It's a dog, is male)

If you are talking about doctor being a property of someone,like his profession, you say "es médico" but if you are refering to the doctor as an entity and not a property of someone you have to use "a":

Es un médico muy famoso (He is a very famous doctor). "Es médico muy famoso" again makes no sense.