**Core Concept**
Gestational trophoblastic disease (GTD) encompasses a spectrum of abnormalities, ranging from hydatidiform moles to invasive mole, choriocarcinoma, and placental-site trophoblastic tumor. The prognosis of GTD depends on various factors, including the type of disease, tumor size, and metastasis to specific organs.

**Why the Correct Answer is Right**
In GTD, metastasis to the lungs is generally associated with a good prognosis. This is because the disease tends to respond well to chemotherapy, and lung metastases are often more amenable to treatment than metastases to other organs, such as the brain or liver. The lungs are also a common site for GTD metastasis, and the disease often presents with pulmonary symptoms, such as shortness of breath or chest pain.

**Why Each Wrong Option is Incorrect**
**Option B:** Brain metastasis is associated with a poor prognosis in GTD, as the disease is often resistant to chemotherapy and the brain is a sanctuary site for tumor cells.
**Option C:** Liver metastasis is also associated with a poor prognosis in GTD, as the disease tends to be more aggressive and the liver is a common site for metastasis from other primary tumors.
**Option D:** Bone metastasis is rare in GTD and is often associated with a poor prognosis, as the disease can be resistant to chemotherapy and bone metastases can cause significant morbidity.

**Clinical Pearl / High-Yield Fact**
In GTD, the presence of high levels of beta-human chorionic gonadotropin (β-hCG) is a marker of poor prognosis, as it indicates a more aggressive tumor. Therefore, monitoring β-hCG levels is an essential part of follow-up care for patients with GTD.

**Correct Answer:** A.