MRCP2-4216

A 14-year-old girl presents to the outpatient clinic with her parents, complaining of small volume haemoptysis and mild breathlessness. Her mother reports that she has had repeated chest infections since childhood and is the smallest of her siblings. The patient’s siblings are all healthy, and they have a pet cat at home. On examination, the patient appears well and is not dyspnoeic, but coarse crackles are heard throughout both lung fields, and she has nasal polyps. The following investigations were conducted:

– Haemoglobin: 113 g/L (130 – 180)
– WCC: 12.9 ×109/L (4 – 11)
– Platelets: 438 ×109/L (150 – 400)
– CRP: 41 mg/L (<10)
– Serum Sodium: 138 mmol/L (137 – 144)
– Serum Potassium: 4.1 mmol/L (3.5 – 4.9)
– Urea: 5.3 mmol/L (2.5 – 7.5)
– Creatinine: 69 μmol/L (60 – 110)
– Bilirubin: 37 μmol/L (1 – 22)
– AST: 49 U/L (1 – 31)
– ALP: 228 U/L (45 – 105)

Spirometry results are as follows:

– FEV1: 1.8L/min (62% predicted)
– FVC: 3.1 L (83% predicted)
– Kco: 72% predicted

CXR shows patchy haziness throughout both lung fields, with tramlines noted at the bases. ECG shows sinus rhythm with no abnormalities.

What is the most appropriate test to confirm the likely diagnosis?