Okay, let's tackle this question. The patient is a 19-year-old with lower abdominal pain, a swollen, painful right knee, fever, and an elevated WBC. The pelvic exam shows adnexal thickening and discharge. So, the key here is to figure out what's going on and what the complications could be if untreated.

First, the symptoms point towards an infection. The pelvic exam findings and discharge suggest pelvic inflammatory disease (PID). The knee symptoms might be reactive arthritis, also known as Reiter's syndrome, which is associated with sexually transmitted infections like chlamydia. The fever and high WBC support an infectious process.

If PID is left untreated, the complications can be severe. The most common ones include infertility due to scarring of the fallopian tubes, ectopic pregnancy, chronic pelvic pain, and septicemia. But wait, the question mentions the knee. Reactive arthritis can develop after an infection, but the main issue here is the pelvic infection. So the key complication would be related to the reproductive system.

Looking at the options, the correct answer should be something like infertility or tubal damage. Let me check the options again. The correct answer given is probably option C: Infertility due to tubal scarring. The other options might include things like knee septic arthritis, which is possible but less likely than the pelvic complications. The knee is a secondary site here, so the main concern is the pelvic infection leading to infertility. The other options might be abscesses or septicemia, but the most common long-term complication of untreated PID is infertility.

**Core Concept**
This case illustrates **pelvic inflammatory disease (PID)** caused by ascending sexually transmitted infections (e.g., *Chlamydia trachomatis*, *Neisseria gonorrhoeae*). Untreated PID can lead to **tubal scarring and infertility** due to inflammation and fibrosis of the fallopian tubes. Reactive arthritis (e.g., Reiter’s syndrome) may occur as a secondary complication.

**Why the Correct Answer is Right**
The patient’s symptoms (lower abdominal pain, adnexal thickening, fever, leukocytosis) and reactive arthritis (swollen knee) strongly suggest PID with a **reactive arthritic response**. If untreated, **tubal scarring** is the most likely complication. Inflammation of the fallopian tubes causes adhesions, obstruction, and impaired fertility. Pathologically, neutrophilic infiltration and fibrosis replace normal tubal tissue, preventing egg transport and increasing ectopic pregnancy risk.

**Why Each Wrong Option is Incorrect**
**Option A:** *Septic arthritis of the knee* is unlikely. The knee is inflamed but reactive, not infected (no purulent fluid or systemic sepsis).
**Option B:** *Abscess formation* in the pelvis is rare without prior surgical intervention or severe obstruction.
**Option D:** *Septicemia* is a rare acute complication compared to chronic tubal damage.

**Clinical Pearl / High-Yield Fact**
PID caused by *Chlamydia* or *Gonorrhea* often presents with **reactive arthritis** (Reiter’s syndrome triad: urethritis, conjunctivitis, arthritis). Untreated PID leads to **infertility in 10-15% of cases

✓ Correct Answer: D. Tubo-ovarian abscess