What is the most likely diagnosis and recommended initial management for a child presenting with prolonged fever, polymorphous rash, abdominal pain, facial edema, leukocytosis, mild proteinuria, and ketonuria?

Kawasaki Disease

Most Likely Diagnosis

This child has Kawasaki disease. The constellation of prolonged fever, polymorphous rash, facial edema (puffy face), abdominal pain, leukocytosis, and mild proteinuria strongly suggests this diagnosis, even though the complete classic criteria may not yet be fully apparent 1.

Diagnostic Assessment

Clinical Criteria Evaluation

Immediately perform a meticulous physical examination to identify the five principal features of Kawasaki disease 1:

• Bilateral bulbar conjunctival injection without exudate or photophobia 1
• Oral mucosal changes: erythematous cracked lips, strawberry tongue, diffuse oral/pharyngeal erythema (no ulcers) 1
• Polymorphous rash (already present): typically maculopapular, truncal with groin accentuation, or erythroderma—never vesicular 1
• Extremity changes: acute erythema/edema of hands and feet with sharp demarcation at wrists/ankles; periungual desquamation appears 2–3 weeks later 1
• Cervical lymphadenopathy: unilateral, ≥1.5 cm diameter in the anterior cervical triangle 1

The "puffy face" (facial edema) described is consistent with the extremity changes seen in Kawasaki disease 1, 2.

Laboratory Findings Supporting Kawasaki Disease

The patient's laboratory results are highly consistent with Kawasaki disease 1, 2:

• Leukocytosis (elevated white cell count) is characteristic, typically >15,000 cells/µL 2
• Mild proteinuria (albumin 30 mg/dL) reflects renal involvement 1
• Sterile pyuria is a common finding that should not be mistaken for urinary tract infection 1, 2
• Ketonuria (160) likely reflects poor oral intake due to fever and illness 2

Immediate Diagnostic Work-Up

Order the following tests immediately 2:

• Inflammatory markers: ESR (expect ≥40 mm/hr, often >100 mm/hr) and CRP (expect ≥3 mg/dL) 2
• Complete blood count with differential: to document leukocytosis, neutrophilia, and assess for anemia 2
• Comprehensive metabolic panel: to check for hypoalbuminemia (≤3.0 g/dL) and elevated ALT 2
• Urgent transthoracic echocardiogram: mandatory to assess for coronary artery involvement (z-score ≥2.5 for LAD or RCA indicates abnormality) 1, 2

Incomplete Kawasaki Disease Consideration

This patient may have incomplete Kawasaki disease, which is common when fewer than 4 of the 5 principal features are present 1. The American Heart Association algorithm mandates echocardiography when fever ≥5 days is accompanied by 2–3 principal features and elevated inflammatory markers 1, 2. Coronary artery abnormalities on echocardiography confirm the diagnosis even with only 3 clinical features 2.

Immediate Management

Treatment Initiation

Begin treatment immediately once the diagnosis is established 1, 3:

• Intravenous immunoglobulin (IVIG): 2 g/kg as a single infusion 1, 3
• High-dose aspirin: 80–100 mg/kg/day divided into four doses 1, 3

Treatment must be initiated within 10 days of fever onset to reduce the risk of coronary artery aneurysm from approximately 25% to about 5% 1, 3. High clinical suspicion permits starting IVIG and aspirin before completing the full echocardiographic evaluation 2.

Monitoring for Complications

Check vital signs every 2–4 hours during the acute phase, watching for signs of Kawasaki Disease Shock Syndrome (KDSS): tachycardia, hypotension, decreased perfusion, or altered mental status 4. Approximately 5% of children develop cardiovascular collapse requiring intensive care 4.

Critical Diagnostic Pitfalls

• Do not dismiss the abdominal pain as unrelated; gastrointestinal symptoms including diarrhea, vomiting, and abdominal pain are common in Kawasaki disease 1, 5
• Do not attribute the proteinuria to urinary tract infection alone; sterile pyuria of urethral origin is characteristic of Kawasaki disease 1, 2
• Do not wait for all five principal features to appear simultaneously; clinical features often emerge sequentially over several days 2
• Do not delay treatment while awaiting laboratory results; if clinical suspicion is high with fever ≥5 days and multiple features, initiate therapy promptly 2

Differential Considerations

While Kawasaki disease is the most likely diagnosis, briefly exclude 5, 6:

• Viral exanthems (measles, adenovirus): typically shorter fever duration, different rash pattern
• Scarlet fever: responds to antibiotics, has sandpaper-textured rash
• Drug reaction: requires medication exposure history
• Other vasculitides (Henoch-Schönlein purpura): different clinical pattern with palpable purpura and joint involvement

The combination of prolonged fever, polymorphous rash, facial edema, abdominal pain, leukocytosis, and proteinuria makes Kawasaki disease the overwhelming diagnostic priority 1, 3.