A 9-year-old girl has had a sore throat for the past 2 days. On physical examination there is pharyngeal erythema with yellowish exudates over swollen palatine tonsils. A Gram stain of the exudate shows gram-positive cocci in chains. She is given penicillin therapy. What is the most likely complication prevented by prompt treatment of this girl?

She has a group A p-hemolytic streptococcal pharyngitis, and the feared complication is an autoimmune response from molecular mimicry to streptococcal M proteins. Rheumatic fever results 2 to 3 weeks later from formation of antibodies directed at endocardium, epicardium, and/ or myocardium (rheumatic heart disease). Poststreptococcal glomerulonephritis may also occur. The pharyngitis is unlikely to spread elsewhere or produce septicemia. Streptococcus pneumoniae is more likely to produce meningitis, otitis, and pneumonitis. Streptococci are unlikely to involve liver.