Which phase of syphilis is associated with severe complications if untreated?

The phase of syphilis associated with severe complications if untreated is the tertiary stage. During this final phase, which can occur years after the initial infection, syphilis can lead to serious health issues affecting multiple organ systems. Manifestations may include damage to the heart, brain, and other vital organs, potentially resulting in life-threatening conditions such as cardiovascular syphilis or neurosyphilis.

While the primary and secondary phases do involve significant health concerns, they do not lead to the catastrophic outcomes seen in the tertiary stage if the infection remains untreated. The potentially latent phase signifies a period where symptoms may not be present, but the infection continues to exist in the body, which could eventually lead to tertiary syphilis if left untreated. Therefore, the tertiary phase is critical due to its association with irreversible complications impacting overall health.