What was the hardest thing you have had to study to date, and how long did it take roughly to get it completely??

Question: What was the hardest thing you have had to study to date, and how long did it take roughly to get it completely??
- Keywords:
  challenge,
  study
Asked by Clodagh on 25 Nov 2020.
- Marcello Valente answered on 25 Nov 2020:
  
  Dear Clodagh,
  for me it has been immunology since it’s quite complex, difficult, and the professor did everything to make it difficult >:(

Question: What was the hardest thing you have had to study to date, and how long did it take roughly to get it completely??