**Core Concept**
Restrictive cardiomyopathy is characterized by impaired ventricular relaxation and reduced diastolic function due to stiffening of the myocardium, often from infiltration, fibrosis, or storage disease. Unlike systolic cardiomyopathies, the primary defect lies in ventricular compliance, not contractility.

**Why the Correct Answer is Right**
Amyloidosis causes extracellular deposition of amyloid proteins, leading to myocardial stiffness and impaired filling. Sarcoidosis results in granulomatous infiltration and fibrosis, particularly in the interstitial myocardium, reducing ventricular compliance. Storage diseases like glycogen storage disease type 1 or Fabry disease involve accumulation of abnormal substrates within cardiac cells, causing progressive myocardial rigidity. All these conditions lead to restrictive physiology, making D the most accurate choice.

**Why Each Wrong Option is Incorrect**
Option A: Amyloid is a correct cause, so it cannot be the only answer.
Option B: Sarcoidosis is a valid cause of restrictive cardiomyopathy and should not be excluded.
Option C: Storage diseases such as Fabry or glycogenosis are well-documented causes of restrictive cardiomyopathy and are not exceptions.

**Clinical Pearl / High-Yield Fact**
Remember: "Amyloid, sarcoid, storage" — all are key causes of restrictive cardiomyopathy. Early diagnosis is vital as they often present with dyspnea and normal or reduced ejection fraction, mimicking other heart diseases.

✓ Correct Answer: D. All of the above