A presumptive sign that would lead the nurse practitioner to suspect pregnancy in an adolescent is:

Rationale:
A. Gonorrhea can present with vaginal discharge and irritation, which can mimic bacterial vaginosis, making it an important condition to consider in the differential diagnosis.
B. Pelvic inflammatory disease typically involves upper genital tract infection with symptoms such as lower abdominal pain, fever, and cervical motion tenderness, rather than isolated vaginal discharge.
C. HPV infection usually presents with genital warts or asymptomatic infection and is not typically confused with bacterial vaginosis.
D. Syphilis presents with characteristic lesions such as chancres or rash, not the thin, gray-white vaginal discharge associated with bacterial vaginosis.

Rationale:
A. Penicillin G benzathine is the treatment for confirmed syphilis, but treatment should not be initiated solely on a reactive RPR without confirmation.
B. Discussing safe sexual practices is important but does not confirm or treat the infection.
C. Vaginal culture is not used to diagnose syphilis.
D. A fluorescent Treponema pallidum antibody-absorption (FTA-ABS) test is used to confirm a positive RPR. The RPR is a non-treponemal screening test that can yield false positives, so confirmation with a treponemal-specific test like FTA-ABS is required before initiating treatment.