**Core Concept**
An interatrial septal defect (IASD) is a congenital heart defect characterized by an opening in the wall of tissue that separates the right and left atria. This defect can lead to various cardiac complications, such as pulmonary hypertension and right-sided heart failure. A thorough evaluation of the cardiac anatomy and function is essential to diagnose IASD and other associated abnormalities.

**Why the Correct Answer is Right**
The best diagnostic tool to declare the case as IASD with other cardiac abnormalities is a Transesophageal Echocardiogram (TEE). TEE provides high-quality images of the cardiac structures, allowing for the detection of even small defects and abnormalities, such as IASD, atrial septal aneurysms, and other cardiac malformations. Additionally, TEE can assess the cardiac function, including the evaluation of the size and function of the ventricles, the status of the valves, and the presence of any shunts or regurgitation.

**Why Each Wrong Option is Incorrect**
* **Option A:** Chest X-ray may show some signs suggestive of IASD, such as increased pulmonary vascular markings, but it is not the best diagnostic tool for this condition.
* **Option B:** Electrocardiogram (ECG) may show some non-specific changes, such as right axis deviation or left atrial enlargement, but it is not sensitive enough to diagnose IASD.
* **Option C:** Cardiac MRI may provide detailed images of the cardiac structures, but it is not as readily available or as quick as TEE for emergency situations.

**Clinical Pearl / High-Yield Fact**
In patients with suspected IASD, it is essential to perform a thorough physical examination, including a careful evaluation of the peripheral pulses, cardiac rhythm, and lung sounds, to identify any signs of cardiac failure or pulmonary hypertension.

**Correct Answer: C. Cardiac MRI.