Is the medicinal cream used to treat anal itching considered impure (najis)? And does it defile underwear, given that it leaves visible traces and is difficult to remove during ablution (istinja'), which affects the performance of acts of worship?

The cream mentioned is pure unless it is made from an impure substance. The basic principle for things is purity, and it is not obligatory to remove it. However, any color or smell you find on clothes due to the emission of impurity necessitates purification for prayer, because the purity of the garment and body from impurity is a condition for the validity of prayer. The presence of impurity does not prevent touching the Mus'haf (Qur'an) if you are in a state of Wudu' (ablution); rather, it prevents prayer. Touching the Mus'haf requires purity from minor ritual impurity.