Medical examinations are part of just about every selection process. A doctor must say that you are medically fit and will be able to perform the essential functions of the job.
What can you expect? Generally medical exams are a known process meaning we know what to expect. Most agencies will specify what the examination will entail. There are usually no surprises. Keep in mind that many agencies are now doing drug screening as part of the medical exam.
If you should fail, some agencies permit a second opinion, but many only give you one chance at passing the medical.
Many of the disqualifications that occur are because the candidate is overweight or has problems with vision.
Want some motivation? Imagine not getting a job you really want because you are a couple of pounds overweight. Knowledge of what you will be subject too during the exam is your best offense.