Just have your observership acceptance letter with you during your visa interview along with other supporting documents. I got B1/B2 visa for observership and I was not even asked to show any document not even the observership acceptance letter during my visa interview. VISA officers know what an observership is but if they ask you to explain then you should be ready with your answer. And even if you get observership at any private clinic in US through contacts then also no need to worry. Just ask the doctor under whom you will be observing to provide you with an observership acceptance letter on their official letterhead and trust me this will serve your purpose equally as good as an official acceptance letter from some university.Also try to travel on the same dates which you mentioned during your visa interview unless there are unavoidable circumstances and when you travel to US you should carry same documents which you had during your visa interview as the immigration officer there at the port of entry can ask for them. I was asked to show the letter of acceptance there at the port of entry in US. Just be honest during this whole process of visa and immigration and answer every question with confidence and honesty. And for confidence its better to practice for these interviews atleast for 1-2 days. Questions frequently asked during these interviews can be found on google. Prepare best and honest answer for it and do practice it before the interview. During my visa interview I was asked that why I am going and for how many months? And what I am currently doing in India and what my Father does because i showed him as my financial support . And its always better to apply for visa during your MBBS or any other degree because visa officer will be convinced by the fact that you have to return back to complete your degree. They just need an assurance that the person will return back. Thats the biggest thing to prove during your interview. And if you are done with your degree and want to apply for the visa then do join some job to show that you can't leave your job as its the only source of income for you (only you ,not your family). Or an alternative is to do observership when you will take CS because at that time there are high chances that you will get the visa easily by showing your exam letter from ECFMG. But still you have to convince them that you will return to your country after your exam and observership. Thats your headache now how to convince them . According to me some job either at a private clinic or at a government hospital serves the purpose.
Bottomline is that just stick to your purpose of applying and never beat around the bush in front of visa officer and immigration officer. You should be able to convince them politely that you will return back to your country and last but not the least , be ready with all the documents that they can ask for.