Complex sentence: During his third year, he began his medical residency at the "Hospital Policlínico San Martín", a medical center that received the most complicated cases from practically all of Buenos Aires Province and brought him into contact with patients for the first time.
Split into simpler sentences: During his third year, he began his medical residency at the "Hospital Policlínico San Martín", a medical center that received the most complicated cases from much of Buenos Aires province. This residency brought him into contact with patients for the first time.