**Core Concept**
Intralobar sequestration is a rare congenital anomaly characterized by a non-functioning, non-communicating portion of lung tissue that receives its blood supply from systemic arteries instead of the pulmonary arteries. This condition often presents with recurrent infections and can be life-threatening if left untreated.

**Why the Correct Answer is Right**
Intralobar sequestration is most commonly seen in the lower lobes of the lungs, particularly in the left lower lobe. This is due to the embryological development of the lung, where the systemic arteries supplying the sequestered segment usually arise from the descending aorta or its branches. The sequestered lung tissue lacks normal bronchial connections and alveoli, leading to recurrent infections and respiratory distress.

**Why Each Wrong Option is Incorrect**
**Option A:** Upper lobe sequestrations are less common, but can occur in a variety of locations, including the upper lobes. However, they are not the most common location.
**Option B:** Posterior mediastinum sequestrations are a type of extralobar sequestration, which is a different condition altogether. They are not typically associated with recurrent infections in the same way as intralobar sequestrations.
**Option C:** Middle lobe sequestrations are extremely rare and are often associated with other congenital anomalies.

**Clinical Pearl / High-Yield Fact**
Intralobar sequestration should be suspected in children with recurrent pneumonia or atelectasis that does not respond to standard antibiotic therapy. Imaging studies such as CT scans or MRI can help confirm the diagnosis.

**Correct Answer:** D. Lower lobe.