**Core Concept**
Lung abscesses are localized collections of pus within the lung parenchyma, often resulting from bacterial infections. They can occur in various lung regions due to different etiologies, such as aspiration, hematogenous spread, or direct extension from adjacent structures.

**Why the Correct Answer is Right**
The least common site of lung abscess is the middle lobe of the right lung. This is because the middle lobe is smaller and less frequently involved in the process of aspiration, which is a common cause of lung abscesses. Aspiration typically occurs in the dependent regions of the lungs, such as the lower lobes, due to gravity.

**Why Each Wrong Option is Incorrect**
**Option A:** The upper lobe of the right lung is actually a more common site for lung abscesses, particularly in the context of staphylococcal pneumonia. This is because staphylococcal pneumonia often presents with cavitation.

**Option B:** The right lower lobe is also a common site for lung abscesses, particularly in the context of aspiration pneumonia. This is because the right lower lobe is a dependent region of the lung, making it more susceptible to aspiration.

**Option C:** The left upper lobe is another common site for lung abscesses, particularly in the context of Staphylococcus aureus pneumonia. This is because Staphylococcus aureus often presents with cavitation in the upper lobes.

**Clinical Pearl / High-Yield Fact**
Lung abscesses are more common in the elderly and in patients with underlying lung diseases, such as chronic obstructive pulmonary disease (COPD). They often present with symptoms of pneumonia, such as fever, cough, and sputum production, but may also have additional symptoms such as weight loss and night sweats.

**Correct Answer:** D.