**Core Concept**
Restrictive cardiomyopathy (RCM) is a condition characterized by the stiffening of the heart muscle, making it difficult for the heart to fill with blood during diastole. This results in impaired cardiac function, often leading to symptoms such as shortness of breath, fatigue, and swelling in the legs.

**Why the Correct Answer is Right**
Restrictive cardiomyopathy can be caused by various conditions that lead to the scarring or stiffening of the heart muscle. These include:
- Amyloidosis: a condition where abnormal proteins, called amyloid, accumulate in the heart muscle, leading to its stiffening.
- Hypertrophic cardiomyopathy: a condition where the heart muscle becomes abnormally thick, which can lead to restrictive filling.
- Sarcoidosis: an inflammatory condition that can cause scarring in the heart muscle.
- Radiation therapy: can cause fibrosis and scarring in the heart muscle.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a known cause of restrictive cardiomyopathy.
**Option B:** This option is incorrect because it is a type of cardiomyopathy that typically presents with impaired systolic function, not restrictive filling.
**Option C:** This option is incorrect because it is a type of cardiomyopathy that typically presents with hypertrophy of the left ventricle, not restrictive filling.

**Clinical Pearl / High-Yield Fact**
Amyloidosis is a well-known cause of restrictive cardiomyopathy, and it can be challenging to diagnose due to its nonspecific symptoms. A high index of suspicion and a thorough physical examination are essential for early diagnosis.

**Correct Answer: D. Dilated Cardiomyopathy**