**Question:** The commonest site of lodgement of a pulmonary embolus is in the territory of

A. Right pulmonary artery
B. Left pulmonary artery
C. Lung parenchyma
D. Pulmonary capillaries

**Correct Answer:** **A. Right pulmonary artery**

**Core Concept:** Pulmonary embolism (PE) is a life-threatening condition where a blood clot obstructs the blood flow in the pulmonary circulation. It can lead to severe consequences such as acute respiratory distress, hypoxia, and right heart strain. The majority of pulmonary emboli, particularly those derived from deep vein thrombosis (DVT), tend to lodge in specific areas of the pulmonary circulation.

**Why the Correct Answer is Right:**
In the human respiratory system, the pulmonary circulation consists of the right and left pulmonary arteries, which branch out from the pulmonary trunk. The right pulmonary artery is larger than the left pulmonary artery due to the larger size of the right ventricle and the increased blood flow it serves. This difference in size is crucial when considering the site of embolus lodgement.

**Why Each Wrong Option is Inaccurate:**
A. Lodgement in the left pulmonary artery is less common due to the smaller size of the left ventricle and reduced blood flow.
B. Pulmonary artery branches into pulmonary lobar arteries, which further branch into segmental and subsegmental arteries. While the embolus can occlude these vessels, the primary site of lodgement is the pulmonary artery trunk or its major branches.
C. Lodgement in the lung parenchyma is incorrect because pulmonary embolism primarily affects the pulmonary circulation, occluding the pulmonary arteries, not the lung tissue itself.
D. Pulmonary capillaries are too small for clot formation and do not represent a site of embolus lodgement.

**Clinical Pearl:**
Understanding the size and anatomy of the pulmonary arteries is essential for diagnosing and treating pulmonary embolism appropriately. In clinical practice, recognizing the right pulmonary artery's larger size can help differentiate PE from other respiratory conditions. Additionally, knowing the correct site of embolus lodgement allows for targeted therapy and improved patient outcomes.