**Core Concept**
The "square root sign" refers to a characteristic echocardiographic pattern observed during ventricular filling, specifically in conditions where there is a marked abnormality in the normal diastolic filling process. This sign is indicative of a specific pathophysiological mechanism that affects the ventricular filling dynamics.

**Why the Correct Answer is Right**
The square root sign is seen in conditions characterized by a restrictive filling pattern, where the ventricular filling is impeded due to increased stiffness of the ventricular wall. This is often observed in conditions such as constrictive pericarditis. The restrictive filling pattern leads to a characteristic M-mode echocardiogram pattern where the early diastolic filling wave (E wave) is significantly reduced, and the late diastolic filling wave (A wave) is significantly increased, giving a "square root" appearance. This is due to the increased stiffness of the ventricle, which restricts the normal diastolic filling process.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the square root sign is not typically associated with conditions that affect the atria, such as atrial fibrillation or atrial septal defects.
* **Option B:** This option is incorrect because the square root sign is not typically associated with conditions that primarily affect the ventricular systolic function, such as heart failure with reduced ejection fraction.
* **Option D:** This option is incorrect because the square root sign is not typically associated with conditions that primarily affect the aortic or mitral valves, such as aortic stenosis or mitral regurgitation.

**Clinical Pearl / High-Yield Fact**
The square root sign is a classic echocardiographic finding in constrictive pericarditis, and its presence should raise suspicion for this condition, especially in patients with a history of heart failure or pericardial disease.

**Correct Answer: C. Constrictive pericarditis**