**Core Concept**
Rheumatic fever, a complication of untreated or inadequately treated streptococcal pharyngitis, can lead to cardiac involvement, including valvular damage and heart failure. This condition is characterized by a migratory polyarthritis, particularly affecting large joints like the elbows and knees.

**Why the Correct Answer is Right**
The patient's symptoms, including pain and swelling in the elbows and knees, along with new mitral regurgitation and tachycardia, are indicative of rheumatic fever. The condition often presents with a migratory polyarthritis, which is a classic symptom. In addition to the joint involvement, rheumatic fever can also cause cardiac complications, including mitral regurgitation due to valve damage. The inflammation caused by rheumatic fever can lead to the formation of Aschoff bodies, which are granulomatous lesions characteristic of the disease.

**Why Each Wrong Option is Incorrect**
* **Option A:** While erythema marginatum (a rash) can be seen in rheumatic fever, it is not the most common cardiac manifestation of the disease. Erythema marginatum is a characteristic rash seen in some cases of rheumatic fever, but it is not as commonly associated with cardiac involvement as other symptoms.
* **Option B:** Pericarditis can be seen in rheumatic fever, but it is not the most likely cardiac manifestation in this case. Pericarditis is inflammation of the pericardium, the sac surrounding the heart, and can be seen in rheumatic fever, but it is not as commonly associated with mitral regurgitation as other symptoms.
* **Option C:** Myocarditis can be seen in rheumatic fever, but it is not the most likely cardiac manifestation in this case. Myocarditis is inflammation of the heart muscle, and while it can be seen in rheumatic fever, it is not as commonly associated with mitral regurgitation as other symptoms.

**Clinical Pearl / High-Yield Fact**
Rheumatic fever is a preventable complication of streptococcal pharyngitis. Early diagnosis and treatment of streptococcal pharyngitis with antibiotics can prevent the development of rheumatic fever.

**Correct Answer:** D.