The nurse practitioner instructs the pregnant adolescent that several diagnostic tests will be performed during prenatal visits. She should return for serum alpha-fetoprotein (AFP) around:

Rationale:
A. A throat culture identifies current streptococcal infection but does not confirm a recent infection that may have triggered glomerulonephritis.
B. An antistreptolysin O (ASO) titer measures antibodies produced in response to a recent group A Streptococcus infection, making it the most reliable test for linking acute glomerulonephritis to a prior streptococcal infection.
C. Erythrocyte sedimentation rate (ESR) is a nonspecific marker of inflammation and cannot confirm recent streptococcal infection.
D. Blood urea nitrogen (BUN), creatinine, and glomerular filtration rate (GFR) assess kidney function but do not indicate the causative pathogen.

Rationale:
A. Retesting at 4 weeks may detect residual nucleic acid from the initial infection and is not recommended for routine follow-up.
B. Six weeks is also too soon for reliable retesting, as residual genetic material from chlamydia may yield false-positive results.
C. Two months is closer, but guidelines recommend a slightly longer interval.
D. Three months following treatment completion is recommended for retesting adolescents diagnosed with chlamydia to detect reinfection, which is common in this age group, and to ensure treatment effectiveness.