The A section in this piece begins with a four bar introduction in the bass line. the right hand begins to play, too, in a simple rhythm but fairly complex melody in measure 5. it is difficult to say what key this piece even though the key signature is indicated (no sharps or flats), and accidentals are all over the place, as is typical in bartok's works. as far as structural phenomena is concerned, there are well placed indications for dynamics (including sFz's) and the texture in the A section is relatively thick because the left hand plays chords while the right hand plays the light melody.
the B section moves slightly faster than what was previously heard. the hands take turns playing eighth notes, so there is constantly an eighth note pulse. the hands take a scalar run together as the B section comes to an end. to me, the section does sound tonally open, but transitions somewhat smoothly into the return of the A section.
When the A section does return, it is not exactly the same as the first exposition melody. the melody in the right hand is much more active with 16th note runs. it does play on the first melody, however.
it is obvious to the listener that the A and B sections are independent from each other, both rhythmically and tonally.