Assumptions:

Users ~10 million worldwide
Exam Subjects : ~= 50
Questions: ~= 100 Questions/subject every questions with 4 options and 1 correct answer
Negative marking is there

Open Questions:

You have to design a system that will do following:

1. On exam completion total score and subject wise score calculation (make sure u consider negative marking).
2. On exam completion Rank calculation.
3. Provision for MOCK tests (users should get hypothetical ranks by comparing previous year results)
4. A pre-exam (with limited users ~10k) to mimic actual exam and then give generalised ranks for more users(in millions).

Design high Level architecture and Database Design and choice.