**Core Concept**
Acute rheumatic fever (ARF) is a complication of untreated or inadequately treated group A beta-hemolytic streptococcal (GABHS) infection, characterized by autoimmune inflammation of the heart, joints, skin, and brain. The condition can progress to rheumatic heart disease, leading to valve damage and potentially life-threatening complications.

**Why the Correct Answer is Right**
Myocarditis, or inflammation of the heart muscle, is a known complication of ARF. In severe cases, it can lead to cardiac failure, arrhythmias, and even death. The immune system's response to the streptococcal infection triggers an autoimmune reaction, causing the body to attack its own heart tissue. This can result in fibrosis and scarring of the heart muscle, compromising its function. The heart's inability to pump blood effectively can lead to inadequate oxygenation of vital organs, ultimately causing the patient's death.

**Why Each Wrong Option is Incorrect**
**Option A:** Central nervous system involvement, such as Sydenham's chorea, is a possible complication of ARF but is not typically a direct cause of death.
**Option B:** Endocarditis, or infection of the heart valves, is a known complication of ARF, but it is not the most likely cause of death in this scenario. While endocarditis can lead to significant morbidity, myocarditis is more directly associated with cardiac failure and death.
**Option D:** Streptococcal sepsis, or a systemic infection caused by the bacteria, is a potential complication of untreated streptococcal infections but is less commonly associated with ARF than myocarditis.

**Clinical Pearl / High-Yield Fact**
In patients with ARF, early recognition and treatment of myocarditis are crucial to prevent cardiac complications and mortality. Clinicians should be vigilant for signs of cardiac involvement, such as chest pain, shortness of breath, and arrhythmias, and promptly initiate treatment with anti-inflammatory medications and antibiotics.

✓ Correct Answer: C. Myocarditis.