If you missed an examination for medical reasons, you should submit a signed medical certificate within seven days to the Examination Section of the Registry. You will likely be permitted to sit the exam at the next available sitting. (Refer to Examination Regulations, Section II, 17-32).
For any other reason, you would normally be obliged to register again for the full course. However, departments, upon being advised of the situation, have other discretionary options that may be applied. Refer to the current issue of the Faculty Handbook, Exams Only Regulations 11.2.