## **Core Concept**
Bronchial cysts, also known as bronchogenic cysts, are congenital anomalies that arise from the abnormal budding of the tracheobronchial tree during embryonic development. They are typically lined by ciliated columnar epithelium and contain mucus or clear fluid. These cysts can occur in the mediastinum or within the lung parenchyma.

## **Why the Correct Answer is Right**
The correct answer can be deduced by evaluating the characteristics of bronchial cysts. Bronchial cysts are indeed **seen in the mediastinum (a)**, which is a correct statement as they often present in this location. They are also known to occur **within the lung (b)**, with estimates suggesting that 50-70% of bronchial cysts occur in the lung parenchyma, making this statement true as well. However, **they are not commonly infected (c)**, although infection can occur, especially if the cyst communicates with the airway or becomes secondarily infected. Lastly, **bronchial cysts are typically unilocular (d)**, not multilocular, which makes option **D** the statement that stands out as not true regarding bronchial cysts.

## **Why Each Wrong Option is Incorrect**
- **Option C:** While bronchial cysts can become infected, especially if they communicate with the airways, stating they are "commonly infected" might not accurately reflect their typical presentation. Infection is a complication rather than a common state.
- **Option D:** This option states that bronchial cysts are multilocular, which is incorrect. Typically, bronchial cysts are unilocular.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that bronchial cysts are congenital anomalies and can be found in both the mediastinum and the lung parenchyma. They are usually lined by ciliated columnar epithelium. A classic presentation might involve an asymptomatic mass found incidentally on imaging, though symptoms can arise due to complications like infection or rupture.

## **Correct Answer:** D. Multilocular