Final answer:
The signs point to an ectopic pregnancy, a medical condition where the embryo implants outside the uterus, often in the uterine tube, which can lead to complications if not treated promptly.
Step-by-step explanation:
The symptoms described in the question suggest a positive Human Chorionic Gonadotropin (HCG) test in the presence of an empty uterus on ultrasound, accompanied by one-sided lower abdominal pain and red vaginal spotting. These signs are characteristic of an ectopic pregnancy, where the embryo implants outside the uterine cavity, often in the uterine tube. Ectopic pregnancies can be notably dangerous if not treated promptly, as they may lead to tubal rupture, causing severe bleeding.
Factors such as scar tissue within the tube from a sexually transmitted bacterial infection can cause an ectopic pregnancy. Tubal pregnancies can cause bleeding, stimulating smooth muscle contractions and potentially expelling the embryo. When ectopic pregnancy is detected early, methotrexate may be administered to inhibit folic acid metabolism, stopping the growth of the embryo. Late diagnosis may necessitate surgical intervention to repair a ruptured uterine tube.