"Exists" should be plural

Question

Your English original is: "the tango is one of the most sensual dances that exists in the world". During the course of the exercise, I felt that "exists" should really be plural, so I put "existen" in my translation answer - and that was accepted as correct. However, your final Spanish version still makes it singular.

Accepted Answer

I would have parsed the sentence as "...the tango is one of [the most sensual dances that exist in the world]."

Within the brackets there is only one plural subject, so I would also have thought that the verb must be plural. I read the article by the RAE, but the examples they give seem to be a bit different from this case.

Answer

Hola David,
We originally admitted both the singular and the plural of the verb because with "construcciones partitivas", i.e. quantifier + de + plural, the RAE seems to accept both. You can consider the subject as a singular subject taking into account the quantifier or you can consider it plural taking into account the plural noun, e.g. "la mayoría de las personas hace/hacen..." or "la mitad de los cuidadanos votó/votaron...". We thought initially that the construction "uno/una de + plural noun" would fall into this category, but having revised it, there seems to be a consensus and we think, at least, most people would use it in the singular form "existe", probably because of the word "uno" which implies the singularity despite the plural noun after "de". We have then omitted the plural from the correct answers, only admitting "existe". 
You can read about this here from RAE. (scroll down until you see 4.8. Sujeto de cuantificador + de + sustantivo en plural.)
concordancia 
Saludos,
Inma