**Core Concept**
Pneumocystis jiroveci is a fungus that primarily affects individuals with compromised immune systems, particularly those with HIV/AIDS. It causes Pneumocystis pneumonia (PCP), a type of opportunistic infection that can be life-threatening.

**Why the Correct Answer is Right**
Pneumocystis jiroveci is a unicellular fungus that belongs to the class Stramenopila. It is characterized by its ability to form cysts, which are resistant to environmental stress and can survive outside the host. The infection occurs when the fungus is inhaled, and it primarily affects the lungs. The pathogenesis of PCP involves the fungus adhering to the alveolar epithelial cells, leading to inflammation and damage.

**Why Each Wrong Option is Incorrect**
**Option A:** Pneumocystis jiroveci is a bacterium. This is incorrect because Pneumocystis jiroveci is a fungus, not a bacterium.

**Option B:** Pneumocystis jiroveci is a common cause of pneumonia in healthy individuals. This is incorrect because Pneumocystis jiroveci primarily affects individuals with compromised immune systems, not healthy individuals.

**Option C:** Trimethoprim-sulfamethoxazole (TMP-SMX) is the first-line treatment for Pneumocystis pneumonia. This statement is actually correct, but since it's an "except" question, we'll consider it as a distractor. However, to maintain the format, we'll address it: This is correct in the context that TMP-SMX is a standard treatment for PCP.

**Option D:** Pneumocystis jiroveci is a type of mycobacterium. This is incorrect because Pneumocystis jiroveci is a fungus, not a mycobacterium.

**Clinical Pearl / High-Yield Fact**
Pneumocystis jiroveci pneumonia can be prevented with prophylactic trimethoprim-sulfamethoxazole (TMP-SMX) in high-risk individuals, such as those with HIV/AIDS. This is a crucial clinical correlation that students should remember.

**Correct Answer: D. Pneumocystis jiroveci is a type of mycobacterium.**