This app sounds similar like a deck of **[Flash Cards][1]**, or Quiz cards. 

With flash cards, the goal is often to work one card after another, keeping the user in a focussed state. They do a burst of exercises, then take a break. It's similar to a sprint. 

Having **Next** and **Prev** links will save them from having to hop back and forth from gallery to detail.

If you give them a close link, they can always go back to the list.



**Keep in-card navigation simple.**

You mentioned having a dropdown to go to a specific card. This assumes they've memorized a title or sequence number. This will get difficult with 50 cards. Plus a dropdown with a long list in mobile gets unwieldy. 


In that case, they can just close the current card and pick from the list, which you said has a preview of the exercise itself.




  [1]: https://en.wikipedia.org/wiki/Flashcard