I took a years' gap from 2018-19 because I was suffering from the deficiency of Vitamin D. Could my visa get rejected due to this?

Hi, There is no reason your visa will be rejected for a health reason. You must also clear a medical test depending on the country. Firstly, focus on submitting a good application to the university or college of your choice; you can apply for the visa only after you have been accepted by the instit