усиленно готовиться к экзамену(значение "усиленно готовиться" является некорректным. To mug up - to quickly try to learn the main facts about a subject, especially before an exam. "Don't just mug up on the key points you need for the exam and hope that you'll scrape through with little effort" - from "English Phrasal Verbs in Use" Bob_cat)