**Core Concept**
Rheumatic fever is an autoimmune inflammatory disease that occurs after an infection with group A beta-hemolytic streptococci (GABHS), typically following streptococcal pharyngitis. The disease can manifest with a variety of clinical symptoms, including carditis, arthritis, chorea, erythema marginatum, and subcutaneous nodules.

**Why the Correct Answer is Right**
The correct answer, **C. ac**, is correct because polyarthritis (Option A) is indeed a manifestation of rheumatic fever, occurring in approximately 50% of patients. The most common valve involvement is indeed the Mitral valve (Option C), which is affected in approximately 70% of patients with rheumatic heart disease. This is due to the valve's unique anatomy, which makes it more susceptible to inflammation and scarring.

**Why Each Wrong Option is Incorrect**
**Option B:** Caused by α hemolytic streptococcus is incorrect because rheumatic fever is caused by group A beta-hemolytic streptococci (GABHS), not α hemolytic streptococcus.

**Option D:** Erythema marginatum is common in face is incorrect because while erythema marginatum is a manifestation of rheumatic fever, it typically occurs on the trunk and limbs, not the face.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that rheumatic fever is a preventable disease through proper treatment of streptococcal pharyngitis with antibiotics, and also through vaccination against streptococcal infections.

**Correct Answer Line**
✓ Correct Answer: C. ac