Hello Aspirant

Irrespective of the chosen B.Sc curriculum and M.Sc curriculum you can pursue your PhD in any branches relevant to your work field without even adhering to the core course. For the PhD you can sit for different entrance exams of very prestigious institutions like Jadavpur University, IIT BHU, Jamia Milia Islamia Institute etc. When you select any other subject than your core, try to choose such a one which atleast has some minute connection with your B.Sc or M.Sc. That will help in your progress,

Good luck!!!