Government & Defence Entrance Exam

Yes. Even you can prepare while pursuing your degree itself. This is the complete list of UPSC Optional subjects for civil services main examination. You have to opt for any one subject. Animal Husbandry and Veterinary Science
Anthropology
Botany
Chemistry
Civil Engineering
Commerce and Accountancy
Economics
Electrical Engineering
Geography
Geology
History
Law
Management
Mathematics
Mechanical Engineering
Medical Science
Philosophy
Physics
Political Science and International Relations
Psychology
Public Administration
Sociology
Statistics
Zoology
Literature of anyone the following language: Assamese, Bengali, Bodo, Dogri, Gujarati, Hindi, Kannada, Kashmir, Konkani, Maithili, Malayalam, Manipur, Marathi, Nepali, Oriya, Punjabi, Sanskrit, Santhali, Sindhi, Tamil, Telugu, Urdu, English. Agriculture.

Educational Qualification: Candidate applying for civil service exams is required to fulfill the following criteria: Candidate must hold a Bachelor's Degree from any of the recognised universities Candidates who have appearedfor the qualifying examination and are awaiting results or those who are yet to appear for the qualifying examination are also eligible for the Preliminary Examination. Such candidates have to produce proof of passing the said examination along with the application for the Main Examination Candidates with professional and technical qualifications recognised by the Government or its equivalent are also eligible to apply Candidates who have passed the final year of MBBS or any Medical Examination but are yet to complete the internship can also appear for the Main Examination. However, they must submit a certificate from the concerned University that they have passed the final professional medical examination.

Every other civil or government service in the country is the specialist service, and each person to some extent reaches the top of hierarchy, but in the end our structure is such that the final say is with the IAS officers. Except for some departments like science and technology, defence research and maybe a few more departments, where the specialist is the Secretary. Railways is one exception (don't know for how long). So IAS officer has in a way become the super generalist. The officer's reach is phenomenal and definitely the best job in goal today from this point of view. The IAS has abnormally high level of diversity in job. Good or bad for the nation is a different issue, but that is a fact. It doesn't seem likely that this will change in a few decades.