If it were so easy for ordinary people to switch specialties, the term "genius" wouldn't exist.
An ordinary doctor, without taking any shortcuts and without wasting time, is about twenty-two years old by the time they graduate from a five-year undergraduate program. If they pursue a professional master's degree, with effort, they can simultaneously complete their residency training. If they opt for an academic master's degree, add another three years for residency training, and by this time, they are about twenty-eight.
After working for a year, they can take the exam for the attending physician qualification.
So, essentially, a doctor is truly ready to be fully utilized by the time they are thirty years old.
Even in such extreme post-apocalyptic circumstances, setting aside those systematic courses and compressing the study time, it still takes more than one or two years to train a mature doctor.
Moreover, the result might not necessarily be stronger than Lin Xia.
