The anterior base of the lung rests on the diaphragm at the midclavicular line at which rib?

• A. Fifth rib
• B. Sixth rib
• C. Seventh rib
• D. Eighth rib

Answer: (B)

The key idea is where the lung’s base contacts the diaphragm along the front of the chest. At the midclavicular line, the anterior part of the diaphragmatic surface of the lung reaches the chest wall at a specific rib level. That level is the sixth rib, so the anterior base of the lung rests on the diaphragm around the sixth rib.

This explains why options that place the border higher, like the fifth rib, or lower or more lateral positions, don’t fit the front view as well. The midclavicular line marks a boundary where the inferior border of the lung sits higher than it does along the midaxillary or posterior lines, which is why the sixth rib is the landmark for this anterior position.