Consider the medical exam as a requisite set to your advantage. You are able to dig up your real health condition through this. You could either confirm here that you are truly healthy or if you have ailments that were undetected before, they would be unraveled for treatment.
The medical exam always comes before the interview. The U.S. Embassy appoints certain accredited clinics or physicians where you may take the medical exam. Although the series of tests included in the medical exam may vary in every Consulate, there are still common denominators for them all.
The common documents necessary for the medical exam are colored 2x2 photos, NVC letter, your valid passport, and medical exam fee. Requirements and amount of payment vary from one country to another. Check with the U.S. Embassy nearest to you.
Remember to keep a copy of the vaccination sheet DS-3025 after you receive your vaccines together with your X-ray CD. Both of these are useful for future immigration transactions.
Stay as healthy as possible and hang on because you are one step closer to the day when you finally get to hold the visa in your hands.