Pulmonary hypertension is a serious condition with multiple underlying aetiologies which require different treatment strategies. We present a case of severe idiopathic pulmonary arterial hypertension in a 20-year-old patient with ongoing breathlessness. She was initially diagnosed with asthma and panic attacks in community care. As the symptoms became progressively worse, she was referred for pulmonary hypertension clinic assessment. Ventilation/perfusion single-photon emission computed tomography (V/Q SPECT) showed grossly abnormal perfusion defects which were mismatched to the ventilation scan, suggestive of chronic thromboembolic disease. However, corroborating computed tomographic (CT) pulmonary angiogram and invasive pulmonary angiography showed no thromboembolic disease. Histological examination of the pulmonary arteries post-mortem showed changes consistent with idiopathic pulmonary arterial hypertension. This case highlighted the clinical challenges in interpreting the investigation results and phenotyping pulmonary hypertension. V/Q SPECT might have a role in visualising the extent of vasculopathies in pulmonary arterial hypertension.