A patient is referred to the neurologist because of ataxia. Neurological examination reveals a loss of proprioception and a wide-based, slapping gate. Magnetic resonance imaging reveals degeneration of the dorsal columns and dorsal roots of the spinal cord. Which of the following organisms is most likely to have caused this pattern of damage?

The findings described are those of tabes dorsalis, a form of teiary syphilis caused by Treponema pallidum. Tabes dorsalis, and other forms of teiary syphilis, are now uncommon in this country, possibly because the common use of antibiotics may "treat" many unsuspected cases of syphilis. Haemophilus influenzae and Neisseria meningitidis can cause meningitis. Neisseria gonorrhoeae causes gonorrhea, which usually does not involve the CNS. Herpes simplex I can cause an encephalitis that typically involves the frontal and temporal lobes.