Acute Rheumatic Fever
A 14-year-old boy presents with 10 days of joint pain that migrated from the right knee to the left ankle and then the right elbow. He had an untreated sore throat 3 weeks ago. The left ankle is swollen and hot. There are painless subcutaneous nodules over both olecranon processes, and the electrocardiogram shows a PR interval of 0.22 seconds. Which investigation would best confirm the underlying aetiology?
- A.Serum C-reactive protein
- B.Antistreptolysin O titreCorrect
- C.Blood cultures
- D.Rheumatoid factor
Explanation
Migratory polyarthritis with subcutaneous nodules, fever and a prolonged PR interval after untreated pharyngitis is acute rheumatic fever, and evidence of preceding group A streptococcal infection is a mandatory part of the modified Jones criteria; a raised or rising antistreptolysin O titre, or anti-DNase B where available, provides that evidence. Acute rheumatic fever is a delayed immune-mediated sequel of group A streptococcal pharyngitis in which antibodies against streptococcal M protein cross-react with cardiac myosin, synovium, skin and basal ganglia, which is molecular mimicry. Diagnosis requires two major criteria, or one major plus two minor, together with proof of antecedent streptococcal infection; this patient has two major criteria, which are polyarthritis and subcutaneous nodules, plus minor criteria of fever, raised acute phase reactants and a prolonged PR interval. Erythrocyte sedimentation rate and C-reactive protein are only minor criteria and are entirely non-specific, so neither confirms the aetiology. Blood cultures are typically negative in rheumatic fever and are used to diagnose infective endocarditis or septic arthritis, which would give a persistently inflamed single joint rather than a migratory pattern. Rheumatoid factor supports rheumatoid arthritis, which produces a symmetrical persistent small joint polyarthritis with morning stiffness. Treatment is high-dose aspirin or naproxen for arthritis, penicillin to eradicate residual organisms, and long-term secondary prophylaxis with benzathine penicillin G to prevent recurrent carditis.
Why each option
- A.
- C-reactive protein is a non-specific minor criterion that reflects inflammation without identifying its cause.
- B.
- Correct. A raised antistreptolysin O titre documents recent group A streptococcal infection, an obligatory element of the Jones criteria.
- C.
- Blood cultures are negative in rheumatic fever and are used to investigate endocarditis or septic arthritis.
- D.
- Rheumatoid factor points to rheumatoid arthritis, which causes symmetrical persistent small joint disease rather than migratory arthritis.
Reference: AHA Revision of the Jones Criteria for the Diagnosis of Acute Rheumatic Fever, Circulation, 2015; Nelson Textbook of Pediatrics, 22nd ed., 2024
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