What is the likely diagnosis and treatment for a 13-year-old boy presenting with swelling and rash two weeks after streptococcal (strep) pharyngitis?

Post-Streptococcal Sequelae: Acute Rheumatic Fever or Post-Streptococcal Glomerulonephritis

This 13-year-old boy presenting with swelling and rash two weeks after streptococcal pharyngitis most likely has acute rheumatic fever (ARF) or post-streptococcal glomerulonephritis (PSGN), both nonsuppurative complications of untreated or inadequately treated Group A streptococcal infection. 1

Likely Diagnoses and Clinical Differentiation

Acute Rheumatic Fever

• ARF typically develops 2-3 weeks after GAS pharyngitis and presents with major manifestations including carditis, polyarthritis (migratory joint swelling), erythema marginatum (characteristic rash), subcutaneous nodules, and Sydenham chorea 1
• The timing (two weeks post-pharyngitis) and presence of both swelling (likely arthritis) and rash strongly suggest ARF 1
• ARF is primarily a disease of children aged 5-15 years, making this 13-year-old boy in the peak age range 1
• The rash in ARF (erythema marginatum) appears as pink-red macules with pale centers that spread outward in a ring shape, typically on the trunk and proximal extremities 1

Post-Streptococcal Glomerulonephritis

• PSGN also occurs 1-3 weeks after GAS pharyngitis and presents with edema (swelling), hematuria, hypertension, and oliguria 2
• The swelling in PSGN is typically periorbital and peripheral edema rather than joint swelling 2
• If the patient has facial/periorbital swelling, dark urine, or decreased urine output, PSGN becomes more likely 2

Immediate Diagnostic Workup

Obtain the following tests immediately:

• Throat culture or rapid antigen detection test (RADT) to document current or recent GAS infection, though may be negative if infection has resolved 1
• Anti-streptolysin O (ASO) titer and anti-DNase B to confirm recent streptococcal infection 1
• Complete blood count, erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP) - elevated in ARF 1
• Urinalysis to check for hematuria and proteinuria (suggests PSGN) 2
• Electrocardiogram and echocardiogram to evaluate for carditis if ARF is suspected 1
• Chest X-ray if respiratory symptoms present to rule out pulmonary involvement 2

Treatment Approach

If Acute Rheumatic Fever is Confirmed:

Primary antibiotic therapy:

• Administer penicillin or amoxicillin immediately to eradicate any remaining GAS, even though this will not alter the course of the current ARF episode 1
• Penicillin V 250-500 mg orally 2-3 times daily for 10 days OR amoxicillin 25-45 mg/kg/day divided every 12 hours for 10 days 3
• A single dose of intramuscular benzathine penicillin G may be preferred to ensure compliance 1

Anti-inflammatory therapy:

• For arthritis without carditis: NSAIDs (ibuprofen or naproxen) provide effective symptom relief 1
• Avoid aspirin in children due to Reye syndrome risk 1
• For carditis: corticosteroids may be indicated (consult pediatric cardiology/rheumatology) 1

Long-term secondary prophylaxis:

• After treating the acute episode, initiate continuous antibiotic prophylaxis to prevent ARF recurrence, as recurrences carry higher risk of severe cardiac damage 1
• Benzathine penicillin G 1.2 million units IM every 3-4 weeks is the most effective prophylaxis regimen 1
• Duration of prophylaxis depends on presence and severity of cardiac involvement: minimum 5 years or until age 21 (whichever is longer) for ARF without carditis; lifelong for severe cardiac disease 1

If Post-Streptococcal Glomerulonephritis is Confirmed:

Antibiotic therapy:

• Treat with penicillin or amoxicillin for 10 days to eradicate GAS, though this does not alter the course of PSGN 3, 2
• Dosing: same as above for ARF 3

Supportive management:

• Fluid and sodium restriction if edema and hypertension present 2
• Antihypertensive therapy if blood pressure elevated 2
• Monitor renal function closely with serial creatinine and urinalysis 2
• Most cases of PSGN resolve spontaneously with supportive care 2

Critical Pitfalls to Avoid

• Do not dismiss swelling and rash as simple allergic reaction or viral exanthem - the two-week timing after strep pharyngitis is pathognomonic for post-streptococcal complications 1
• Do not delay echocardiography - subclinical carditis may be present even without obvious cardiac symptoms, and early detection is crucial for prognosis 1
• Do not use corticosteroids for uncomplicated streptococcal pharyngitis, but they may be indicated for ARF with carditis 1
• Do not confuse this presentation with Kawasaki disease - Kawasaki typically presents with prolonged fever (≥5 days), conjunctivitis, mucositis, and occurs in younger children (typically <5 years) 1
• Recognize that even if the patient was treated for the initial pharyngitis, inadequate treatment (wrong antibiotic, insufficient duration, poor compliance) can still lead to ARF or PSGN 1
• Do not assume negative throat culture rules out post-streptococcal sequelae - the acute infection may have resolved, making ASO and anti-DNase B titers essential 1

Why This Matters for Morbidity and Mortality

ARF can cause permanent cardiac valve damage (rheumatic heart disease), leading to lifelong disability and premature death 1. The initial episode of ARF is the critical window for preventing recurrent episodes through secondary prophylaxis, which dramatically reduces the risk of progressive cardiac damage 1. PSGN, while usually self-limited, can rarely progress to pulmonary-renal syndrome with diffuse alveolar hemorrhage, which carries high mortality 2. Prompt recognition and appropriate treatment of the initial streptococcal pharyngitis with a full 10-day course of penicillin or amoxicillin is the only proven method to prevent these devastating complications 1, 3.