What secondary cause of pulmonary hypertension involves obstruction?

The secondary cause of pulmonary hypertension that involves obstruction is chronic recurrent pulmonary embolism. This condition occurs when blood clots (emboli) repeatedly block the pulmonary arteries. The obstruction in the pulmonary circulation increases resistance to blood flow, leading to elevated pulmonary arterial pressure, which characterizes pulmonary hypertension.

In the case of chronic recurrent pulmonary embolism, the repeated episodes of obstruction can lead to pulmonary vascular remodeling and long-term pulmonary hypertension. This contrasts with other conditions that can cause pulmonary hypertension without direct obstruction to blood flow. For instance, chronic obstructive pulmonary disease can lead to hypoxic vasoconstriction but does not primarily involve an obstructive mechanism. Left-sided heart failure causes pulmonary hypertension primarily due to increased left atrial pressure rather than obstruction within pulmonary arteries. Interstitial lung disease involves lung tissue changes leading to pulmonary hypertension but again does not relate directly to obstructive processes.

Thus, chronic recurrent pulmonary embolism stands out as the condition most directly characterized by obstruction leading to pulmonary hypertension.