**Core Concept**
P. carini (now known as Pneumocystis jirovecii) is a fungus that primarily infects the lungs of immunocompromised individuals, such as those with HIV/AIDS or undergoing chemotherapy. This opportunistic pathogen causes a life-threatening infection known as Pneumocystis pneumonia (PCP).

**Why the Correct Answer is Right**
P. carini infects primarily the lungs, specifically the alveoli, where it causes inflammation and damage to the epithelial lining. The fungus invades the alveolar space and forms cysts, which rupture to release trophic forms that adhere to the alveolar epithelium, leading to a severe inflammatory response. This results in symptoms such as dyspnea, non-productive cough, and hypoxemia.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because P. carini does not primarily infect the skin or soft tissues.
**Option B:** This option is incorrect because P. carini is not typically associated with gastrointestinal infections.
**Option C:** This option is incorrect because while P. carini can cause systemic symptoms, it primarily infects the lungs.

**Clinical Pearl / High-Yield Fact**
P. carini pneumonia is a classic opportunistic infection in HIV/AIDS patients, often presenting with symptoms of respiratory distress and hypoxemia. It is essential to diagnose PCP promptly, as treatment with trimethoprim-sulfamethoxazole can be life-saving.

**Correct Answer:** D. Lungs.