**Core Concept**
The sudden death of a young individual during physical activity, coupled with chamber and septum hypertrophy at autopsy, suggests a condition that affects the heart's structure and function, particularly in relation to the myocardium and septum.

**Why the Correct Answer is Right**
The most likely diagnosis is Hypertrophic Cardiomyopathy (HCM), a genetic disorder characterized by abnormal thickening of the heart muscle (myocardium), particularly in the septum. This thickening can lead to obstruction of blood flow, arrhythmias, and sudden cardiac death, especially during intense physical activity. HCM is often caused by mutations in genes encoding sarcomeric proteins, which are essential for normal myocardial contraction and relaxation.

**Why Each Wrong Option is Incorrect**
**Option A:** While arrhythmogenic right ventricular cardiomyopathy (ARVC) can cause sudden death, it is typically characterized by fibrofatty replacement of the myocardium rather than hypertrophy.
**Option B:** Dilated cardiomyopathy (DCM) involves enlargement and thinning of the heart muscle, which is the opposite of the hypertrophy seen in this case.
**Option C:** Restrictive cardiomyopathy (RCM) is characterized by stiffening of the heart muscle, leading to impaired diastolic function, but it does not typically cause sudden death in young individuals during physical activity.

**Clinical Pearl / High-Yield Fact**
Hypertrophic cardiomyopathy is the leading cause of sudden cardiac death in young athletes, and it often presents with a family history of similar cases. A detailed family history and genetic testing may be necessary to diagnose HCM in some cases.

**Correct Answer: C. Dilated cardiomyopathy (DCM) involves enlargement and thinning of the heart muscle, which is the opposite of the hypertrophy seen in this case.