If you have a valid medical reason, talk to the doctor if he is willing to do the necessary paperwork. In this case you can have a pending status. You wont get a F or a W, but have to retake the part, which you couldnt complete earlyer. Even IINS /USCIS does not takes action on you and your Student status can be maintained that way. However, as mentioned you have to produce a doctors certificate, which wont come till your doctor says he will do it for you.