create online "diagnostic" questionnaire with results based on choices
Maybe this helps: Module for decision tree / guided entry. There is nothing which appears do the job out of the box. So you need to evaluate the modules if they full fill your needs. I am going to check following modules to build a so called decision tree:


Decision Tree
D.A.I.L.
Core Book
Outline Designer
Choose Your Own Adventure
Hierarchical Select
Conditional Fields
... to be continued ;)