UPSC

UPSC study material is wide but you can just focus on prime important material
polity -laxmikant polity and NCERT IX-XII
History
India's Struggle for Independence Bipan Chandra
Indian Art and Culture by Nitin Singhania
NCERT XI history
NCERT XII (Modern Indian History)
Geography
Certificate Physical Geography G C Long
NCERT VI X (Old Syllabus)
NCERT XI, XII (New Syllabus)
World Atlas (Orient Black Swan)
Economics
Indian Economy by Nitin Singhania
Economic Development & Policies in India Jain & Ohri
NCERT XI
International Relations
NCERT XII (Contemporary World Politics)
Current Affairs
CSAT
Tata McGraw Hill CSAT Manual
Verbal & Non-Verbal Reasoning R S Aggarwal
important daily news analysis.
These are essential study material for upsc preparation

Dear aspirant, Glad to meet you. The UPSC optional subjects list consists of 48 subjects including Medical Science. The Medical Science syllabus for IAS Exam focuses on the candidates understanding of the basic concepts and application of knowledge to problems faced by patients. The topics included in this subject are related to basic medical knowledge, general medicine and community medicine. Candidates should choose Medical Science as an optional subject in UPSC Mains exam when they have very high competence and a strong interest in the subject so that they can easily prepare for the exam. Candidates who have studied Medical Science in their undergraduate degree or who work as doctors, biomedical engineers, nurses or pharmacists can take up this optional. Hope it helps.

Firstly, you should know about English is not compulsory for UPSC interview. Aspirants writing their Main examination in English language, but their interview medium can be Hindi or such languages. During form filling they ask for your medium of interview. So, you can apply for Hindi also. It doesn't matter which language you speak and how you speak, it is basically what you speak, what is your thought process and how do you think about a particular issue as your reactions are more important than the language. But if you want to improve your English and become fluent during an interview You should speak slowly. You should use simple sentences. And don't bother if you're making grammatical mistakes don't go back to fix them. The interview board will not bother you. They will listen to you patiently.

The Union Public Service Commission UPSC administers the Combined Defence Services CDS exam. I assume you want to compare the Civil Service and CDS. Both are reputed government jobs but are unique in the nature of work. Here is a brief comparison given between the two services:
Civil Services:
1. It includes services such as IAS IPS, and IFS. These are responsible for policy making and implementation.
2. Civil servants maintain public law and order. These services are comparatively less adventurous than CDS.
3. It is mostly a non-uniformed service except for IPS and IRS (C&CE).
4. Civil servants mostly deal with civilians and therefore live close to civilian areas.
5. Civil servants do not have perks such as a canteen, but their salary is usually on par with other group A services. CDS:
1. The CDS exam is generally conducted for defence roles in the Indian Army, Airforce, and Navy.
2. It deals with Indian security and therefore is more adventurous. It can directly expose you to the frontline.
3. The CDS is a well-known uniformed service.
4. CDS does not involve much civilian exposure and officers mostly stay in cantonment areas.
5. CDS officers have additional perks, like canteen service. Therefore, both services are good, and it depends on your aptitude to choose which one you want to choose. If you want an adventurous life that directly involves protecting the country, join the CDS. But if you want to work in the service of people, join the civil service, such as the IAS.