If a 25-year-old female presents with severe right-sided pelvic pain and vaginal bleeding, what is the best initial intervention?

The best initial intervention for a 25-year-old female presenting with severe right-sided pelvic pain and vaginal bleeding is a serum quantitative pregnancy test. This choice is critical because, in young women of childbearing age, pelvic pain and abnormal bleeding can often indicate a possible pregnancy-related issue, such as an ectopic pregnancy, miscarriage, or other gynecological emergencies.

A serum quantitative pregnancy test is both sensitive and specific; it can provide immediate information about the presence of a pregnancy as well as its viability. A positive test would indicate that the patient is indeed pregnant, which then necessitates further evaluation for potential complications like an ectopic pregnancy. Conversely, a negative result can guide the clinician toward other diagnoses.

While other options, such as a pelvic ultrasound, could be necessary for further evaluation in the case of confirmed pregnancy or suspicion of specific gynecological conditions, they would not serve as the immediate first step. A complete blood count (CBC) can help assess for anemia or infection but is not as immediately diagnostic in this context. Follicle-stimulating hormone (FSH) would not be relevant in an acute assessment of pelvic pain and bleeding in the setting described.