What is the most likely diagnosis in a young woman with five days of fever and morbilliform rash who then develops cough, diarrhoea, hypoxemia, tachypnea and bilateral infiltrates on chest X‑ray?

Most Likely Diagnosis: Measles Pneumonia

The most likely diagnosis is measles pneumonia, given the characteristic presentation of fever with morbilliform rash followed by respiratory deterioration with bilateral infiltrates, cough, diarrhea, and hypoxemia in a young woman. 1, 2

Key Diagnostic Features Supporting Measles

Classic Clinical Progression

• Measles pneumonia characteristically appears during the rash period (typically 3-4 days after fever onset), which matches this patient's 5-day timeline before respiratory deterioration 1, 3
• The combination of fever, morbilliform rash, cough, and gastrointestinal symptoms (diarrhea) is highly characteristic of measles 3
• Respiratory symptoms develop during or shortly after rash appearance, with cough being present in 82% of measles pneumonia cases 1

Radiographic Pattern

• Bilateral infiltrates on chest X-ray are the hallmark finding, distinguishing measles from bacterial pneumonia which typically shows lobar consolidation 4, 5
• CT imaging (if performed) would likely show centrilobular nodules in ground-glass opacity (63-73%), ground-glass attenuation (63%), bronchial wall thickening (88.5%), and interlobular septal thickening 1, 6, 7
• These CT findings, particularly the interlobular septal thickening, are measles-specific and not seen in mycoplasma or other atypical pneumonias 7

Clinical Severity Indicators

• Hypoxemia occurs in 91% of measles pneumonia cases and is a key feature 1
• The rapid deterioration over 2 days with tachypnea and hypoxemia is consistent with measles pneumonia progression 2
• Young unvaccinated adults are at particular risk, with 16.9% developing pneumonia during recent outbreaks 2

Critical Differential Considerations

Why Not Primary Influenza Viral Pneumonia?

• Primary influenza viral pneumonia typically develops within the first 48 hours of fever onset, not after 5 days 8, 4
• Influenza lacks the characteristic morbilliform rash that preceded this patient's respiratory symptoms 8
• Influenza presents with bilateral interstitial infiltrates predominantly in mid-zones, but the rash is the key distinguishing feature 4

Why Not Secondary Bacterial Pneumonia?

• Secondary bacterial pneumonia typically develops 4-5 days after initial symptom onset during early convalescence, but presents with lobar consolidation rather than bilateral infiltrates 9, 4, 5
• The bilateral infiltrate pattern argues against bacterial etiology, which characteristically shows focal lobar consolidation 4, 5

Why Not SARS or Other Viral Pneumonias?

• SARS presents with fever and cough but rash is notably absent in SARS cases 8
• The morbilliform rash is pathognomonic for measles and not seen in SARS, influenza, or most other viral pneumonias 3

Immediate Management Priorities

Supportive Care

• Provide supplemental oxygen for hypoxemia and monitor for progression to respiratory failure 2
• Correct dehydration and nutritional deficiencies, particularly given the diarrhea 3
• Monitor closely for severe respiratory impairment, as measles pneumonia in young adults can cause sudden, unpredictable death even in previously healthy individuals 2

Specific Interventions

• Administer vitamin A supplementation, which is a cornerstone of measles management 3
• Consider empiric antibiotics covering S. pneumoniae, S. aureus, and H. influenzae if secondary bacterial superinfection cannot be excluded, though the bilateral infiltrate pattern suggests primary viral process 9, 5
• Obtain serologic confirmation with measles IgM and IgG antibodies 2

Critical Pitfalls to Avoid

• Do not dismiss measles in young adults – recent outbreaks demonstrate significant morbidity and mortality in unvaccinated young adults, with two deaths reported in previously healthy young women 2
• Do not wait for Koplik's spots – these buccal mucosal lesions may not be present or may be missed 3
• Do not assume good prognosis – while measles pneumonia in previously healthy patients often resolves within 6 days, severe cases requiring ICU admission occur in young adults, and death can be sudden and unpredictable 1, 2
• Implement immediate infection control measures – measles is highly contagious via respiratory transmission and requires airborne precautions 3