Q: Does Venus exhibit retrograde motion? (1 mark)
A: No. This is because Venus orbits the Sun and not the Earth.
The first part is correct: Venus does not exhibit retrograde motion. But the explanation is incorrect: the reason Venus doesn't exhibit retrograde motion is because it's closer to the Sun than we are. Mars for example also orbits the Sun and not the Earth, but does exhibit retrograde motion.
Do I award 1 mark or 0? On the one hand, for obvious reasons, the grading scheme only covers whether the student said "yes" or "no". Based on that, I should award 1 mark. Further, if the student hadn't written the incorrect explanation, then the answer is perfect, and it feels wrong to penalize the student for going beyond what the question asks for.
On the other hand, the explanation is clearly incorrect and the student should've known the correct explanation (it's part of the curriculum). It also feels wrong to award full marks for semi-incorrect answers. For example, if the student had written something silly such as "This is because Venus is made of Swiss cheese", do I still award 1 mark?!
Ideally, I'd award 0.5 marks, but for various reasons fractional scores aren't permissible.