Apex domains cannot have CNAME records.
Question and response
Question
dig medical.uns.ac.rs. NS
Answer
medical.uns.ac.rs. 5 NS ns2.uns.ac.rs.
medical.uns.ac.rs. 5 NS ns1.uns.ac.rs.
Authority
The response has no authority records.
Additional
ns1.uns.ac.rs. 2 A 147.91.173.4
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0