What is the most likely diagnosis for a young woman with sudden breathlessness and normal blood gases?

In this scenario, the most likely diagnosis is vocal cord dysfunction. This condition can often present with sudden breathlessness, and it is characterized by erratic movements of the vocal cords during inhalation, making it difficult for air to flow properly. Importantly, patients with vocal cord dysfunction can maintain normal blood gases, as their primary issue is not a failure in gas exchange but rather an obstruction in the airway due to improper function of the vocal cords.

Other options may present with different clinical features or findings. Asthma typically would show abnormal blood gases during an exacerbation due to bronchial constriction and obstruction, leading to hypoxia or hypercapnia. Hyperventilation syndrome might involve changes in blood gases, usually indicating respiratory alkalosis, and symptoms may not present quite suddenly without other symptoms. Pulmonary embolism classically causes issues such as low oxygen saturation and alterations in blood gases as a result of impaired blood flow and ventilation-perfusion mismatch.

Thus, given the sudden onset of breathlessness with normal gas exchange, vocal cord dysfunction stands out as the most fitting diagnosis in this context.