A 50 year old man presented with a one week history of general malaise, worsening shortness of breath, and a cough productive of green sputum. He had also noticed an itchy rash on his trunk and face. There was no relevant medical history and he took no regular medications. He was a non-smoker.

On examination the patient had a widespread symmetrical rash over his face, trunk, and proximal limbs; this consisted of vesicles, pustules, and crusted lesions. He was afebrile, dyspnoeic at rest with a respiratory rate of 36 breaths/min, and tachycardic (pulse 140 beats/min) with a normal blood pressure. Auscultation of his chest revealed vesicular breath sounds bilaterally; the rest of the examination was unremarkable.

Arterial blood gas analysis on air (table 1) demonstrated the patient was profoundly hypoxic. A chest radiograph (fig 2) demonstrated diffuse infiltrates across both lung fields.

QUESTIONS

1. What is the diagnosis?

2. How might the diagnosis be confirmed?

3. What is the differential diagnosis of findings on the chest radiograph?