In a child who had a 4‑day viral upper respiratory tract infection with clear nasal discharge, fever, and sneezing, and then three days after symptom resolution developed purulent nasal discharge and frontal tenderness, what is the most likely diagnosis?

Acute Bacterial Sinusitis

The most likely diagnosis is B - Acute Bacterial Sinusitis, specifically presenting as a "worsening course" or "double-sickening" pattern.

Clinical Reasoning

This child's presentation is a textbook example of acute bacterial sinusitis complicating a viral upper respiratory tract infection. The key diagnostic feature here is the biphasic illness pattern: initial viral URI symptoms that resolved, followed by new onset of purulent nasal discharge and frontal tenderness 3 days after symptom resolution 1.

Why This Meets Criteria for Acute Bacterial Sinusitis

The American Academy of Pediatrics and IDSA guidelines define three distinct presentations of acute bacterial sinusitis in children 2, 1:

1. Persistent illness: Symptoms lasting ≥10 days without improvement
2. Worsening course ("double-sickening"): New onset or worsening of nasal discharge, cough, or fever after initial improvement from a viral URI (typically occurring 5-6 days into illness) 2, 1
3. Severe onset: High fever (≥39°C/102.2°F) with purulent nasal discharge for at least 3-4 consecutive days at illness onset

This child fits the "worsening course" presentation - the viral URI lasted 4 days and resolved, then 3 days later (day 7 of total illness) new symptoms emerged with purulent discharge and facial tenderness 1.

Why Not the Other Options

Option A (Viral URTI) is incorrect because:

• Viral URIs typically resolve within 5-7 days 1
• While purulent nasal discharge can occur transiently during uncomplicated viral URIs, it appears mid-illness and then clears 1
• The biphasic pattern with symptom resolution followed by new purulent symptoms is characteristic of bacterial superinfection, not viral illness 2, 1
• Frontal tenderness suggests sinus involvement beyond simple viral inflammation 1

Option C (Allergic Rhinosinusitis) is incorrect because:

• Allergic rhinitis typically presents with clear (not purulent) nasal discharge 1
• Associated symptoms would include pruritic eyes/nose, sneezing, and pale nasal mucosa 1
• The acute onset following a viral URI with fever makes infection far more likely than allergy 1
• No mention of allergic triggers, seasonal pattern, or atopic history

Clinical Pearls

Common Pitfalls to Avoid

• Don't dismiss purulent discharge as "just part of a cold" - while purulent discharge can occur in viral URIs, the timing and pattern matter. When it appears after initial improvement, suspect bacterial sinusitis 2, 1.

• The "double-sickening" pattern is highly specific - this worsening after initial improvement strongly suggests bacterial superinfection rather than the natural course of viral illness 2, 1, 3.

• Imaging is NOT indicated for diagnosis in uncomplicated cases - the diagnosis is purely clinical based on stringent criteria 1.

Management Implications

According to guidelines, children with a worsening course should receive antibiotic therapy 1:

• First-line: Amoxicillin with or without clavulanate 1, 4
• Standard dose: 45-90 mg/kg/day of amoxicillin component 4
• Duration: 7-10 days 2

Approximately 8% of viral URIs in young children are complicated by acute bacterial sinusitis 5, 4, making this a common and clinically important diagnosis to recognize.