**Core Concept**
Gestational trophoblastic neoplasia (GTN) is a group of rare, malignant tumors that originate from the trophoblastic cells of the placenta. The FIGO (International Federation of Gynecology and Obstetrics) staging system is used to classify the extent of disease in GTN, which is crucial for determining the prognosis and treatment plan.

**Why the Correct Answer is Right**
The correct staging of GTN in the presence of lung metastasis is FIGO stage IV. This is because GTN is considered advanced when it metastasizes to distant organs, such as the lungs. Stage IV GTN has the poorest prognosis among all the stages, and treatment often involves chemotherapy and radiation therapy. The presence of lung metastasis indicates that the cancer has spread beyond the uterus, which necessitates a higher stage classification.

**Why Each Wrong Option is Incorrect**
**Option A:** FIGO stage I GTN is limited to the uterus and has not spread to distant organs, making it an incorrect choice.
**Option B:** FIGO stage II GTN involves the uterus and the uterine cervix, but not distant organs, so it's also an incorrect choice.
**Option C:** FIGO stage III GTN involves the uterus, uterine cervix, and one or both ovaries, but not distant organs, making it an incorrect choice.

**Clinical Pearl / High-Yield Fact**
The FIGO staging system for GTN is crucial for determining the prognosis and treatment plan, and it's essential to accurately stage GTN to provide optimal care for patients.

**Correct Answer:** D