**Core Concept**
Pulmonary embolism (PE) occurs when a blood clot lodges in the pulmonary arteries, obstructing blood flow to the lungs. The site of embolus lodgement is crucial for understanding the clinical presentation and management of PE.

**Why the Correct Answer is Right**
The commonest site of lodgement of a pulmonary embolus is in the left upper lobe due to the anatomy of the pulmonary arteries. The left pulmonary artery is longer and more tortuous than the right, making it more susceptible to clot lodging. Additionally, the left upper lobe receives blood from the left main pulmonary artery, which is the most common site of embolus formation. The clot typically lodges at the bifurcation of the left main pulmonary artery into the upper and lower lobe arteries, leading to infarction of the left upper lobe.

**Why Each Wrong Option is Incorrect**
**Option A:** The right lower lobe is not the most common site of lodgement due to the anatomy of the pulmonary arteries. The right lower lobe receives blood from the right lower lobe artery, which is not as prone to clot lodging as the left upper lobe.
**Option B:** The right upper lobe is not the most common site of lodgement, despite its larger size. The right upper lobe receives blood from the right upper lobe artery, which is not as susceptible to clot lodging as the left upper lobe.
**Option C:** The left lower lobe is not the most common site of lodgement, as it receives blood from the left lower lobe artery, which is not as prone to clot lodging as the left upper lobe.

**Clinical Pearl / High-Yield Fact**
The clinical presentation of a pulmonary embolus can vary depending on the location of the clot. A clot lodged in the left upper lobe may present with sudden onset dyspnea, pleuritic chest pain, and hemoptysis. It is essential to consider the anatomy of the pulmonary arteries when interpreting imaging studies and managing patients with suspected PE.

✓ Correct Answer: D. Lt. upper lobe