Can I improve this Quiz Model?

Anybody could tell me if I can improve the following model of a quiz app? The purple tables are the pivot ones. To see the relationships you have to click on the double arrow button.

You can comment/fork it here: http://www.laravelsd.com/share/hEYbzi

Screenshot:

https://www.dropbox.com/s/3292k9ppviz0y13/Screenshot%202015-08-21%2023.38.18.png?dl=0

Planned Features

• Storing of quiz results under each user
• Previous quiz scores can be viewed on category (Knowledge Area) page
• Success rate for each category (there will be more can be monitored on a progress page
• Multiple choice question type
• Display an optional image alongside the question

Thank you so much!