**Core Concept:**
Interventricular septal hypertrophy refers to the thickening of the wall between the two ventricles of the heart, particularly the right and left ventricles. This condition is often caused by increased cardiovascular demand, such as in athletes.

**Why the Correct Answer is Right:**
The correct answer, Dilated Cardiomyopathy (DCM), is a heart muscle disorder characterized by an enlarged heart and weakened heart muscle. In this context, the athlete's sudden cardiac arrest and interventricular septal hypertrophy are indicative of increased cardiovascular demand due to intense physical activity. DCM is a probable diagnosis as it leads to the dilation of heart chambers and the compensatory hypertrophy of the interventricular septum.

**Why Each Wrong Option is Incorrect:**
A. Hypertrophic Cardiomyopathy (HCM): This condition involves the thickening of the heart muscle, which is different from the dilation seen in DCM and the compensatory hypertrophy of interventricular septum in DCM.
B. Arrhythmogenic right ventricular cardiomyopathy (ARVC): This condition primarily affects the right ventricle, not the interventricular septum, and it is not directly related to intense physical activity.
C. Marfan's syndrome: This condition involves the connective tissue and causes aortic root dilation, not interventricular septal hypertrophy.

**Clinical Pearl:**
In athletes, sudden cardiac arrest due to hypertrophic cardiomyopathy (HCM) or arrhythmogenic right ventricular cardiomyopathy (ARVC) may occur due to unmasking by intense physical activity, as the increased cardiac workload can cause arrhythmias or worsening of heart muscle dysfunction. It is essential for medical professionals to consider these conditions in high-performance athletes presenting with cardiovascular issues to ensure timely diagnosis and management.

**Correct Answer:** Dilated Cardiomyopathy (DCM)