**Core Concept**
Infertility is a complex condition that can result from various factors, including infections, anatomical abnormalities, hormonal imbalances, and genetic disorders. Infections play a significant role, especially in sexually transmitted infections (STIs), which can damage the reproductive organs and disrupt fertility.

**Why the Correct Answer is Right**
Chlamydia and gonorrhea are two common STIs that can cause infertility in both men and women. Chlamydia can ascend to the upper genital tract, causing pelvic inflammatory disease (PID), which can lead to tubal damage and scarring, ultimately resulting in infertility. Gonorrhea can also cause PID and tubal damage, in addition to causing epididymitis in men, leading to impaired sperm function. Mycoplasma and pneumococcus are not typically associated with infertility.

**Why Each Wrong Option is Incorrect**
* **Option A:** Chlamydia is indeed a common cause of infertility, making this option incorrect because it's not the only correct answer.
* **Option B:** Gonorrhea is also a common cause of infertility, similar to Chlamydia, so this option is incorrect for the same reason.
* **Option C:** Mycoplasma is not typically associated with infertility, making this option incorrect.
* **Option D:** Pneumococcus is a bacterium that causes pneumonia and other respiratory infections, not infertility, making this option incorrect.

**Clinical Pearl / High-Yield Fact**
Remember that STIs, particularly Chlamydia and gonorrhea, can cause significant damage to the reproductive organs, leading to infertility. Early diagnosis and treatment are crucial to prevent long-term consequences.

**Correct Answer: A. Chlamydia**