This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
Carbon monoxide diffusion capacity is decreased in all, except: 
Select and explain. 
A. Emphysema, B. Primary pulmonary hypertension, C. Alveolar haemorrhage, D. Infiltrative lung disease
C: Alveolar haemorrhage
Answer: C: Alveolar haemorrhage 
Explanation: Flows remain normal, as does the diffusion capacity of the lung for carbon monoxide (DlCO), unless obstructive sleep apnea (which often accompanies obesity) and associated chronic intermittent hypoxemia have induced pulmonary arterial hypertension,in which case DLCO may be low.