**Core Concept**
Infective endocarditis is a bacterial infection of the heart valves, characterized by fever, heart murmur, and positive blood cultures. The diagnostic criteria for infective endocarditis involve a combination of clinical, laboratory, and echocardiographic findings to confirm the diagnosis.

**Why the Correct Answer is Right**
ESR (Erythrocyte Sedimentation Rate) is a non-specific marker of inflammation and is not a diagnostic criterion for infective endocarditis. It can be elevated in a wide range of conditions, including infections, autoimmune diseases, and malignancies, making it a non-specific and unreliable indicator of infective endocarditis. In contrast, positive blood cultures, positive ECG findings (such as Q-waves or ST-segment changes), and rheumatoid factor can be associated with infective endocarditis, although they are not definitive diagnostic criteria.

**Why Each Wrong Option is Incorrect**
**Option A:** Rheumatoid factor can be associated with infective endocarditis, particularly in patients with a history of rheumatic heart disease or those with a positive rheumatoid factor titer. It is not a definitive diagnostic criterion, but it can be a useful ancillary test.
**Option C:** Positive blood culture is a key diagnostic criterion for infective endocarditis, as it confirms the presence of a bacterial infection. Blood cultures should be drawn before antibiotics are administered.
**Option D:** Positive ECG findings, such as Q-waves or ST-segment changes, can be associated with infective endocarditis, particularly in patients with a history of cardiac disease.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the diagnosis of infective endocarditis requires a combination of clinical, laboratory, and echocardiographic findings. A positive blood culture is a critical diagnostic criterion, and echocardiography can help identify vegetation or abscesses on the heart valves.

**✓ Correct Answer: B. ESR. ESR is not a diagnostic criterion for infective endocarditis.**