**Core Concept**
Acute rheumatic fever (ARF) is a complication of untreated or inadequately treated group A beta-hemolytic streptococcal (GABHS) pharyngitis, leading to autoimmune inflammation of the heart, joints, skin, and brain. The disease primarily affects children and adolescents, with a higher incidence in developing countries.

**Why the Correct Answer is Right**
ARF is a post-streptococcal autoimmune response, triggered by molecular mimicry between the bacterial antigens and human cardiac tissues. The disease occurs most commonly in children aged 5-15 years, who are more likely to develop anti-streptococcal antibodies and have a higher risk of cardiac involvement. The peak incidence is observed in developing countries, where access to healthcare is limited, and GABHS infections are more common.

**Why Each Wrong Option is Incorrect**
**Option A:** Infants and young children are at higher risk of severe GABHS infections, but they are not the most common age group for ARF.
**Option B:** Adults can develop ARF, but the incidence is significantly lower than in children and adolescents.
**Option C:** Developing countries have a higher incidence of ARF due to limited access to healthcare and higher rates of untreated GABHS infections, but the disease is not exclusive to these regions.

**Clinical Pearl / High-Yield Fact**
ARF is a preventable disease through prompt and adequate treatment of GABHS pharyngitis with antibiotics, such as penicillin, and secondary prevention with long-term penicillin prophylaxis in high-risk patients.

**Correct Answer: C. Developing countries.