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Theodor O. Diener discovered the Potato Spindle Tuber Viroid ("PSTVd"), the first viroid ever identified, in 1971. PSTVd is a small, circular RNA molecule. Dr. Diener discovered that the pathogen causing potato spindle tuber disease is not a virus, as previously believed, but a much smaller, free RNA molecule.

1. Viroids are DNA molecules.
2. Viroids are protein molecules.
3. Viroids are lipid molecules.
4. Viroids are RNA molecules.

Correct answer: Viroids are RNA molecules.

Solution

PSTVd was identified as a viroid, and viroids are infectious agents composed only of RNA. The passage explicitly states it is a small, circular RNA molecule, which matches the correct choice.

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