Final answer:
The most likely cause of the patient's present condition is Treponema pallidum, which is the bacterium responsible for syphilis, a sexually transmitted infection.
Step-by-step explanation:
The most likely cause of the patient's present condition is Treponema pallidum, which is the bacterium responsible for syphilis, a sexually transmitted infection. The patient's symptoms of fever and swollen lymph nodes after visiting New Mexico could be indicative of a primary or secondary syphilis infection.
It is important for the doctor to test for Treponema pallidum in order to accurately diagnose and treat the patient's condition.