**Core Concept**
Restrictive cardiomyopathy (RCM) and constrictive cardiomyopathy (CCM) are two distinct conditions affecting the heart's pumping function. RCM is characterized by stiff ventricular walls, which impede diastolic filling, whereas CCM is caused by external compression of the heart, often due to pericardial disease.

**Why the Correct Answer is Right**
In restrictive cardiomyopathy, the ventricles become stiff and non-compliant, impairing diastolic filling. Constrictive cardiomyopathy, on the other hand, occurs when the heart is compressed by a thickened pericardium, which restricts diastolic filling. In some cases, these two conditions can coexist, particularly when a patient has a pericardial disease that causes CCM and also leads to ventricular fibrosis and stiffness, resulting in RCM.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because RCM and CCM have distinct pathophysiological mechanisms and do not typically coexist.
**Option B:** This option is incorrect because while both conditions can lead to diastolic dysfunction, they are caused by different underlying factors and do not occur together.
**Option C:** This option is incorrect because it is not a recognized condition where RCM and CCM coexist.

**Clinical Pearl / High-Yield Fact**
When differentiating between RCM and CCM, clinicians should focus on the presence of external compression (e.g., pericardial disease) and the characteristic "square root sign" seen on diastolic filling patterns in CCM.

**Correct Answer: D.**