What is a major danger of prolonged untreated secondary syphilis?

Prolonged untreated secondary syphilis can indeed lead to tertiary syphilis, which is a major concern in the progression of the disease. Secondary syphilis is characterized by systemic dissemination of the Treponema pallidum bacteria, leading to a variety of symptoms such as skin rashes and mucous membrane lesions. If left untreated, the infection can progress to tertiary syphilis, which may occur years later and can result in severe complications, including damage to the heart, blood vessels, and central nervous system. This stage of syphilis can have debilitating effects and can be life-threatening, highlighting the importance of timely treatment.

The other options do not accurately reflect the consequences of untreated secondary syphilis. Acne and enhancing immunity are not linked to the disease's progression, while the notion of being self-limiting contradicts the established fact that untreated syphilis can lead to more severe health issues later on.