Final answer:
The recommended position for postural drainage of the right middle lobe posterior lung segment is the Left Side-Lying position. By lying on the left side, gravity helps move secretions from the right lung to the larger airways for clearance.
Step-by-step explanation:
For postural drainage of the right middle lobe - posterior lung segment in an adult, the recommended position is B) Left Side-Lying. This position allows gravity to assist in draining mucus from the right middle lobe's posterior segment. When the patient is placed on their left side, the right lung is positioned superiorly, allowing secretions to move towards the larger airways where they can be more easily cleared. This is particularly important as it involves the systemic circulation from the lungs, which then leads to the azygos vein. The azygos vein is critical for draining blood from the thoracic structures such as intercostal veins, esophageal veins, bronchial veins, and other veins in the mediastinal region, eventually leading to the superior vena cava.