All are causes of pulmonary hypertension except,

Ans. is 'a' i.e., Hyperventilation There are three main causes of pulmonary hypertension:-Cardiac diseasesRespiratory diseasePulmonary thromboembolic diseaseCardiac diseasesLead to pulmonary hypertension by increasing pulmonary blood flow (e.g., large left to right shunt) or by increasing pulmonary venous pressure (e.g., L. V. failure).In Respiratory diseases,The predominant mechanism for the pulmonary hypertension is an increase in resistance to pulmonary blood flow arising from perivascular parenchymal changes coupled with pulmonary vasoconstriction due to hypoxia.In pulmonary thromboembolic disease,Clots in pulmonary vessels increase resistance to blood flow causing pulmonary hypertension.Now about other options mentioned in the question.Alveolar hyperventilationPulmonary hypertension is seen in Alveolar hypoventilation (not hyperventilation).In patients with alveolar hypoventilation the primary pathogenic mechanism responsible for pulmonary hypertension is alveolar hypoxia potentiated by respiratory acidosis. (alveolar hypoxia leads to constriction of pulmonary vessels).Regardless of the etiology, whether pulmonary hypertenstion will occur in patients with alveolar hypoventilation depends on whether there is sufficient alveolar and arterial hypoxia to raise pulmonary arterial pressure considerably.In sleep apnea syndrome seen in obese individualsSevere arterial hypomexia occurs during sleep due to hypoventilation (apnea). This causes pulmonary hypertension during sleep. After some point of time, the pulmonary hypertension may continue even during the day time.AnorexiantsPulmonary hypertension has been associated with anorexiants."A causal relationship has been established between exposure to several anorexigens including aminorex and fenfluramine and development of pulmonary artery hypertension. "