## **Core Concept**
The core concept being tested here is the diagnosis of a **Hydatidiform mole**, also known as a molar pregnancy, which is an abnormal pregnancy characterized by the abnormal growth of trophoblasts. The investigation of choice for detecting such a condition involves imaging techniques.

## **Why the Correct Answer is Right**
The correct answer, **Ultrasound**, is the investigation of choice for detecting a Hydatidiform mole. Ultrasound imaging is highly sensitive and specific for identifying the characteristic "snowstorm" appearance of a molar pregnancy. This appearance is due to the presence of multiple echogenic foci (representing the hydropic villi) within the uterus. Ultrasound helps in early detection and management of the condition, reducing the risk of complications such as gestational trophoblastic neoplasia.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While **X-ray** can show an enlarged uterus, it cannot provide the detailed information needed to diagnose a molar pregnancy and exposes the fetus to radiation.
- **Option B:** **CT scan** provides more detailed images than ultrasound but is not the first-line imaging modality for suspected molar pregnancy due to radiation exposure and similar diagnostic capability to ultrasound for this specific condition.
- **Option C:** Although **MRI** can be used in pregnancy for certain indications, it is not the initial investigation of choice for suspected Hydatidiform mole due to its higher cost, limited availability, and the fact that ultrasound is highly effective for this diagnosis.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that a Hydatidiform mole is often suspected clinically due to symptoms like **excessive nausea**, **uterine size larger than dates**, and **vaginal bleeding** during early pregnancy. However, definitive diagnosis is made by ultrasound, which shows the characteristic "snowstorm" appearance.

## **Correct Answer:** . Ultrasound