As someone else pointed out, this is a piece, not a scale. As such, it's allowed to use accidentals and chords from outside the home key. Altered chords, borrowed chords, key changes, etc. are all common in music, and many require extra accidentals not found in the key signature.
To understand where your D# sharp comes from, you would need to do a harmonic analysis: figure out what chord or key the D# belongs to, and what its function is. To do that, you need to understand not just scales, but chords, progressions, keys and modulations. It sounds like you need a little more study and experience in these areas in order to really understand where the extra accidentals in a tune come from.
In the meantime, "play the ink", as a teacher of mine used to say, and try to play it like a pro.
|