Kakumei no Hi

Kei is a normal–if very pretty–young boy in high school.
One day, he falls ill and is taken to the hospital.
They find, much to everyone's surprise, that "he" actually has a female genotype even though "his" phenotype is male!! A little bit of surgical work solves the problem, and cool male hero Kei becomes innocent female heroine Kei.
And what a female! "He" was cute before, but she is a beauty now.