Which organism is responsible for Lyme disease?

Borrelia burgdorferi is the causative agent of Lyme disease, a tick-borne illness primarily transmitted through the bite of infected Ixodes ticks. This organism is a spirochete, which is a type of bacteria that is characterized by its spiral shape.

Lyme disease often presents with an initial localized erythema migrans rash, which is typically referred to as a "bull's-eye" because of its distinctive appearance. As the disease progresses, it can lead to more severe systemic symptoms, including neurological manifestations and arthritis if left untreated.

The other organisms listed are responsible for different diseases: Leptospira spp. is associated with leptospirosis, a zoonotic disease; Corynebacterium diphtheriae causes diphtheria, a respiratory illness; and Cryptococcus neoformans is known for causing cryptococcal meningitis, particularly in immunocompromised individuals. Therefore, the specificity of Borrelia burgdorferi for Lyme disease makes it the correct answer.