A 28-year-old woman presents with severe lower abdominal pain and fever. What is the most likely diagnosis?

In the case of a 28-year-old woman presenting with severe lower abdominal pain and fever, the most likely diagnosis is pelvic inflammatory disease (PID). PID commonly results from the ascent of bacteria from the cervix and vagina into the upper genital tract, often following sexually transmitted infections, such as chlamydia or gonorrhea. The symptoms you mentioned—severe lower abdominal pain and fever—are classic indicators of PID, which can also present with additional symptoms like vaginal discharge and irregular menstrual bleeding.

The other options do not align as closely with the presenting symptoms. Uncomplicated cystitis primarily causes symptoms related to urination, such as dysuria and frequency, and typically does not present with severe abdominal pain and fever. Candida vulvovaginitis usually presents with itching, discharge, and discomfort in the vulvar area rather than significant abdominal pain. Primary syphilis typically starts with a painless ulcer (chancre) at the site of infection and is not characterized by lower abdominal pain or fever.

Given these details, the presentation aligns most closely with pelvic inflammatory disease.