Good point and thanks for finding that. I'll add it to the accepted translations now. Of course, as in English, we could very well use it without "an" such as if we were thinking about the situation. It would need more context to define it otherwise.
There is a typo in one of the correct answers :) When I wrote 'An interesting situation' Duoling corrected me: 'You have a typo in your answer. AnInteresting situation.' And I can't report it.