Which of the following is least associated with relapsing fever?

Syphilis is least associated with relapsing fever because it is primarily caused by Treponema pallidum, which does not lead to the recurrent febrile episodes characteristic of relapsing fever. Relapsing fever is typically caused by certain spirochetes that are known to exhibit the ability to evade the immune system, leading to cycles of fever.

Borrelia species are well-known causes of relapsing fever due to their capacity for antigenic variation, which allows them to cause multiple episodes of fever. Leptospira is associated with a different clinical presentation but is not typically characterized by relapsing fever. Tick-borne pathogens, such as certain Borrelia species, also contribute to relapsing fever. Therefore, while there may be certain associations between leptospirosis and febrile illnesses, it is syphilis that definitively stands apart from the other options, as it does not involve the recurring nature of febrile episodes prevalent in relapsing fever conditions.