**Core Concept**
A hydatidiform mole, also known as a molar pregnancy, is a rare abnormal pregnancy characterized by the growth of a non-viable fertilized egg. The diagnosis of hydatidiform mole is crucial to prevent potential complications such as hemorrhage, infection, and malignant transformation.

**Why the Correct Answer is Right**
The diagnosis of hydatidiform mole is primarily made through ultrasound examination, which shows a characteristic "snowstorm" pattern due to the presence of multiple small, echogenic vesicles. **Ultrasound** is the most sensitive and specific method for diagnosing hydatidiform mole. Additionally, **beta-hCG levels** are often elevated in hydatidiform mole, and serial measurements may help monitor the regression of the mole after evacuation.

**Why Each Wrong Option is Incorrect**
**Option A:** **X-ray** is not a primary diagnostic tool for hydatidiform mole, as it may not show the characteristic ultrasound findings and can expose the patient to radiation.

**Option B:** **CT Scan** is not necessary for the diagnosis of hydatidiform mole, as it provides no additional information compared to ultrasound and exposes the patient to radiation.

**Option C:** **MRI** is not typically used for the diagnosis of hydatidiform mole, as it is more expensive and provides no additional information compared to ultrasound.

**Clinical Pearl / High-Yield Fact**
In cases of suspected hydatidiform mole, it is essential to measure beta-hCG levels serially after evacuation to ensure regression of the mole and to detect any potential complications.

**Correct Answer:** A. Ultrasound.