Hola fenixpollo,This isn't entirely true, fenixpollo. Only in the U.S.(I'm pretty certain), and then only in the fields of law (abogado), medicine (médico) and dentistry (dentista) must you have a BA (bachelor of arts) or BS (bachelor of science) degree in SOME field BEFORE attending the graduate schools of law, medicine or dentistry. In all other fields that I know of, the bachelor's degree in that field gives you the "license" to practice in that field. In the cases of law, medicine and dentisty, since there is no bachelor's degree in those fields in the U.S, naturally a bachelor's degree is not the equivalent of a licenciatura because there is NO bachelor's degree available in those fields.
In all other fields, a bachelor's degree gives you the right to practice that field. A bachelor of science in engineering can practice as an engineer (ingeniero) as soon as he has the degree. He does not have to have any additional years of education or practical experience. The same is true for all other fields in which a bachelor's degree is granted. Your brother, the cardiologist, could have immediately begun working as an engineer when he received his bachelor of science in engineering. He chose not to and then went to medical school to get his graduate degree of doctor of medicine. He could then have immediately begun practicing as a doctor (médico), but he chose to go on to post-graduate studies to become a specialist, in this case a cardiologist.
This was a course of education unique to the U.S. (I believe). I don't think that these sorts of well-defined (in the U.S.) steps are considered the norm in most other countries.
Discúlpeme, pero mi español no fue suficiente para este explicación.
Saludos desde Mazatlán