Final answer:
The most likely explanation for the woman's symptoms is ectopic pregnancy.
Step-by-step explanation:
The most likely explanation for the 28-year-old woman's symptoms is a condition called ectopic pregnancy. Ectopic pregnancy occurs when a fertilized egg implants and grows outside the uterus, typically in the fallopian tube. This can cause abdominal pain, spotting, and an elevated beta-hCG level. The mild left lower quadrant tenderness and positive pelvic exam findings further support this diagnosis.