**Core Concept**
Hydatidiform mole, also known as a molar pregnancy, is a rare abnormality of early pregnancy characterized by the abnormal growth of trophoblastic cells and the formation of a non-viable embryo. The condition is usually diagnosed in the first trimester, and its presentation can be influenced by hormonal changes and the presence of abnormal trophoblastic cells.

**Why the Correct Answer is Right**
The commonest presentation of hydatidiform mole is vaginal bleeding, which typically occurs between 8-12 weeks of gestation. This bleeding is often accompanied by an enlarged uterus and can be preceded by a missed period. The hormonal changes associated with the abnormal trophoblastic cells can lead to an overproduction of human chorionic gonadotropin (hCG), which can cause the uterus to become enlarged. The presence of abnormal trophoblastic cells can also lead to the formation of a non-viable embryo, which can cause the uterus to become tender and enlarged.

**Why Each Wrong Option is Incorrect**
**Option A:** Abdominal pain is not the most common presentation of hydatidiform mole, although it can occur, especially if the mole is large or if there is a rupture of the uterus.
**Option B:** Anemia is not a direct presentation of hydatidiform mole, although it can occur if the bleeding is heavy.
**Option C:** Fever is not typically associated with hydatidiform mole, unless there is a secondary infection.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that vaginal bleeding is a common presentation of hydatidiform mole, and a missed period can be a precursor to this condition. A high index of suspicion is necessary to diagnose this condition early, especially in women with a history of previous molar pregnancies or a family history of gestational trophoblastic disease.

**Correct Answer:** A. Vaginal bleeding.