**Core Concept**
Aerial thromboembolism, also known as pulmonary embolism, occurs when a blood clot breaks loose, travels through the bloodstream, and lodges in one of the pulmonary arteries. The clinical presentation of pulmonary embolism can vary depending on the location and size of the embolus, as well as the patient's overall health status. The lungs are divided into five main lobes, with the middle lobe being the least commonly affected.

**Why the Correct Answer is Right**
The middle lobe is the least commonly affected due to its unique anatomy, which includes a smaller size and a more vertical orientation compared to the other lobes. This makes it less prone to obstruction by emboli, which tend to lodge in the more horizontal and larger pulmonary arteries. The middle lobe's smaller size and lower blood flow may also contribute to its relative protection from embolic events. The right middle lobe is typically smaller compared to the left, which is another reason it is less frequently affected.

**Why Each Wrong Option is Incorrect**
**Option A:** Upper lobe - While the upper lobes are more commonly affected by emboli, they are not the least commonly affected.
**Option B:** Lower lobe - The lower lobes are also commonly affected by emboli, particularly in the context of chronic thromboembolic disease.
**Option C:** Apical segment of the lower lobe - This area is commonly affected by emboli, particularly in the context of chronic thromboembolic disease.

**Clinical Pearl / High-Yield Fact**
The middle lobe's relative protection from embolic events makes it a useful landmark for clinicians when interpreting chest radiographs in patients with suspected pulmonary embolism.

**Correct Answer:** C.