Ahh gotcha.
To obtain an MD in America its 4 year of university and 4 years of residency.
In America, to obtain an MD, you have to first obtain a 4-year Bachelor degree with a concentration in Pre-med, which covers all the necessary sciences for admission into med school (typically 2 semester of each of the following: biology, chemistry (Organic + Inorganic), and physics).
After you obtain your Bachelors Degree in Pre-Med, you then apply to Med School, but must take your MCAT's first.
Once you obtained a decent enough score (35 or higher) on the MCAT's and have your 4-year BS degree (with a good GPA 3.75 or higher), you typically can get into a good Medical School.
Medical School is then 4 years long.
Once you finish Medical School, you match into a particular residency. The most common one to match with is an internal medicine residency, which is a 3-year long residency.
After you complete your internal medicine residency, you can always opt to specialize in something like lets say Cardiology. Cardiology then requires a 3-year fellowship. Once you complete that, you are considered a Cardiologist.
So, in summary, assuming you want to become a Cardiologist (Medical Doctor Specializing on the Heart), it will take you this long:
4 years (Undergrad pre-med) + 4 years (Med School) + 3 years (Internal Medicine Residency) + 3 years (Cardiac Fellowship) = 14 years total from the time you finish high school.
If you already have a Bachelors degree with all the prerequisite sciences and have taken the MCAT's, you can then get into Med School and you can end up at the same end in 10 years (4yrs-med school + 3 yrs internal med resid + 3 yrs cardiac fellowship = 10yrs).
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P.S. Not every residency takes 3 yrs. Some residencies can take as little as 3 years, while others can be up to 5 yrs long.
Example:
Internal Medicine = 3 year residency
Anesthesia = 4 year residency
Orthopedic Surgery = 5 year surgical residency
** Since things like Cardiac & Gastroenterology are sub-specialties of internal medicine, they typically require 3 extra years of what's called a fellowship
ON TOP of the initial 3 year internal medicine residency. So, if you want to become a gastroenterologist, after you finish med school, you have to do 6 extra years of training (3 years of internal medicine + 3 years of Gastro Fellowship).