What is the differential diagnosis for a patient with a new erythematous (red) rash on the head spreading to the face, accompanied by slight pain and preceded by a 2-day cough?

Differential Diagnosis: Red Rash on Head/Face with Preceding Cough

Most Likely Diagnosis: Measles (Rubeola)

The clinical presentation of a red rash starting on the head and spreading downward to the face, preceded by a 2-day cough, is classic for measles, which characteristically begins with a prodromal cough followed by a cephalocaudal rash progression. 1, 2

Key Diagnostic Features of Measles

• Prodromal phase: Cough, coryza (runny nose), and conjunctivitis (the "3 Cs") typically precede the rash by 2-4 days 1, 2
• Rash characteristics: Erythematous maculopapular rash that begins on the face/head and spreads downward in a cephalocaudal pattern over 3-4 days 3, 4
• Koplik spots: Small white spots on buccal mucosa (pathognomonic if present) appear 1-2 days before rash 2, 3
• Fever: High fever typically accompanies or precedes the rash 5, 3

Critical Alternative Diagnoses to Exclude Immediately

Rocky Mountain Spotted Fever (RMSF)

RMSF must be excluded urgently because it carries 5-10% mortality if untreated, and the rash classically begins on wrists/ankles before spreading centrally—opposite to this patient's presentation. 6

• The rash in RMSF typically starts peripherally (wrists, ankles, forearms) and spreads centrally, eventually involving palms and soles 6
• Rash appears 2-4 days after fever onset, not before cough 6
• Involvement of palms and soles suggests serious rickettsial infection requiring immediate doxycycline 6, 5
• This diagnosis is less likely given the head-to-face progression pattern 6

Meningococcemia

Any petechial rash pattern with systemic toxicity requires immediate hospitalization and empiric antibiotics due to rapid progression and high mortality. 5, 3

• Petechial or purpuric rash (not simply erythematous) suggests meningococcemia 5, 3
• Systemic toxicity (fever, altered mental status, hypotension) mandates immediate medical attention 5
• The described "slight pain" and lack of mentioned systemic toxicity makes this less likely 5

Pertussis with Secondary Rash

Pertussis should be considered given the 2-day cough prodrome, though rash is uncommon (5-21% of cases) and typically maculopapular or purpuric, not the primary presenting feature. 6

• Pertussis presents with persistent cough lasting >2 weeks with paroxysms, post-tussive vomiting, or whooping sound 6, 7
• The 2-day cough duration is too short for typical pertussis presentation 6
• Rash, when present, is not the dominant clinical feature 6
• Nasopharyngeal culture and paired sera for pertussis antibodies are diagnostic if suspected 7, 8

Diagnostic Algorithm

Immediate Assessment

1. Check vital signs immediately: Look for fever, tachycardia (≥100 bpm), tachypnea (≥24 breaths/min) to assess severity 7, 8
2. Examine rash morphology carefully: Determine if maculopapular, petechial, or vesicular 2, 3, 4
3. Assess for palm and sole involvement: This suggests rickettsial or serious bacterial infection 6, 5
4. Look for Koplik spots: Examine buccal mucosa for pathognomonic white spots of measles 2, 3

Key Historical Elements

• Vaccination history: Unvaccinated or incompletely vaccinated individuals are at high risk for measles 6, 1
• Recent travel or exposure: Contact with confirmed measles cases or travel to endemic areas 1, 2
• Tick exposure: Recent outdoor activities in endemic areas for RMSF 6
• Medication review: ACE inhibitors can cause cough but not rash 7

Laboratory Evaluation

• Measles IgM antibody: Diagnostic for acute measles infection 1, 2
• Chest radiograph: Only if vital signs abnormal, respiratory distress, or concern for pneumonia complication 7, 8
• Complete blood count: Leukopenia suggests viral etiology; thrombocytopenia suggests rickettsial disease 6

Critical Red Flags Requiring Immediate Action

• Petechial or purpuric rash pattern: Suggests meningococcemia or RMSF—requires immediate hospitalization 6, 5
• Systemic toxicity: Fever with altered mental status, hypotension, or respiratory distress mandates emergency care 5
• Rapidly spreading rash: Progressive deterioration warrants prompt medical evaluation 5
• Palm and sole involvement: Indicates serious rickettsial or bacterial infection requiring immediate antibiotics 6, 5

Common Pitfalls to Avoid

• Do not dismiss measles in vaccinated individuals: Vaccine immunity can wane, and breakthrough infections occur 6, 1
• Do not wait for complete triad: The classic fever-cough-rash triad may not be complete at initial presentation 6, 3
• Do not assume viral illness is benign: Measles can cause serious complications including pneumonia and encephalitis 1, 2
• Do not delay treatment for RMSF if suspected: Mortality increases significantly without early doxycycline therapy 6