**Core Concept:** Coronary artery disease (CAD) is a condition characterized by the narrowing or blockage of the coronary arteries, which supply blood to the heart muscle. Diagnosing CAD is crucial for appropriate management and treatment of affected individuals.

**Why the Correct Answer is Right:**
Coronary artery calcium (CAC) score is a non-invasive, quantitative method used to assess the burden of atherosclerosis in the coronary arteries. It is based on computed tomography (CT) imaging, which measures the amount of calcium deposited within the arterial plaques. A higher CAC score indicates a greater likelihood of obstructive coronary artery disease (CAD) and increased cardiovascular risk.

**Why Each Wrong Option is Incorrect:**
A. Calcium score is not a direct measure of CAD, but rather an indirect assessment of atherosclerosis burden. While CAD is associated with increased calcium deposition, the calcium score may be influenced by factors other than CAD, such as age, sex, and calcium intake.

B. A normal electrocardiogram (ECG) does not rule out CAD, as ECG changes may not always correlate with the severity of CAD. Furthermore, ECG abnormalities can be seen in other cardiac conditions unrelated to CAD.

C. Echocardiography is a non-invasive imaging modality used to evaluate cardiac function and structure. However, it does not directly assess the coronary arteries or provide information about atherosclerosis burden or the presence of CAD.

D. Angiography is an invasive procedure involving the insertion of a catheter into the coronary arteries for visualization. It provides detailed information about the coronary anatomy but is invasive, expensive, and associated with potential complications.

**Clinical Pearl:** Although a single test cannot definitively diagnose CAD, the combination of multiple non-invasive tests like CAC scoring, ECG, echocardiography, and stress tests can provide a comprehensive assessment of coronary artery disease and cardiovascular risk. In clinical practice, an integrated approach using these tests can aid in identifying patients who would benefit from coronary angiography or other invasive investigations.