**Core Concept**
Trophozoite infections are caused by protozoan parasites, which can be transmitted through various routes, including sexual intercourse. This question is testing the student's knowledge of the common modes of transmission for a specific trophozoite infection.

**Why the Correct Answer is Right**
The correct answer is Toxoplasma gondii, which is a protozoan parasite that can be transmitted through sexual contact with an infected person, particularly through the fecal-oral route. However, the most common mode of transmission is through cat feces contaminated with oocysts. When an infected person handles cat litter or comes into contact with contaminated feces, they can ingest the oocysts, which can then infect them. In rare cases, Toxoplasma gondii can be transmitted through sexual contact, particularly if the infected person has a weakened immune system.

**Why Each Wrong Option is Incorrect**
* **Option A:** Trypanosoma cruzi is primarily transmitted through the bite of an infected triatomine bug, also known as a kissing bug.
* **Option B:** Giardia lamblia is typically transmitted through contaminated water or food, not through sexual intercourse.
* **Option C:** Leishmania species are transmitted through the bite of an infected sandfly, not through sexual contact.

**Clinical Pearl / High-Yield Fact**
Toxoplasma gondii can cause congenital infection if a pregnant woman is infected for the first time, which can lead to serious complications for the fetus, including miscarriage, stillbirth, and birth defects.

**Correct Answer:** D. Toxoplasma gondii