The definition of gonorrhea is "STI caused by Neisseria gonorrhoeae." Option C is the right choice.
Gonorrhea is a sexually transmitted infection (STI) caused by the bacterium Neisseria gonorrhoeae.
Option A is incorrect as Trichomonas vaginalis is a different protozoan parasite causing another STI called trichomoniasis.
Option B is incorrect as Treponema pallidum is the bacterium causing syphilis, another STI with different symptoms and pathology.
Option D is inaccurate as gonorrhea does not primarily cause painful blisters, although it can lead to other complications.
Therefore, the correct definition of gonorrhea is STI caused by Neisseria gonorrhoeae.
Option C is the right choice.