**Core Concept**
Acute pelvic inflammatory disease (PID) is an infection of the female reproductive organs, primarily caused by sexually transmitted pathogens such as Neisseria gonorrhoeae and Chlamydia trachomatis. The risk factors for PID are multifactorial, including sexual behavior, reproductive anatomy, and contraceptive use.

**Why the Correct Answer is Right**
Women with a monogamous partner who has undergone vasectomy are at lower risk for PID. This is because vasectomy significantly reduces the risk of transmission of sexually transmitted infections (STIs) from the male partner to the female partner. Vasectomy is a highly effective form of male sterilization, with a failure rate of less than 1% when performed correctly. The absence of STIs in the male partner reduces the risk of PID in the female partner.

**Why Each Wrong Option is Incorrect**
**Option A:** Menstruating teenagers who have multiple sex partners are at increased risk for PID due to their age, sexual behavior, and potential exposure to STIs.
**Option B:** IUD users may be at increased risk for PID, especially if they have an existing STI or if the IUD is inserted in an unsterile environment. However, this option is not the correct answer as it is still a risk factor for PID.
**Option D:** A previous history of acute PID is a significant risk factor for future episodes of PID. This is because women with a history of PID are more likely to have underlying anatomical or immunological factors that contribute to the development of PID.

**Clinical Pearl / High-Yield Fact**
It's essential for clinicians to ask about the number of sexual partners and the use of condoms in patients presenting with symptoms of PID. This information can help identify women at higher risk for PID and guide targeted prevention and treatment strategies.

**✓ Correct Answer: C. Women with monogamous partner who had vasectomy**