Where is friction rub most commonly heard?

Friction rub is a specific type of lung sound that results from the inflammation of the pleurae, the membranes surrounding the lungs. This sound is typically described as a grating or inspirational crunching and is most commonly heard in areas where there is significant movement or expansion of the lungs during respiration.

The areas of greatest thoracic expansion correspond to the locations where the pleurae are likely to come into contact with one another, creating friction as the surfaces slide against each other during breathing. This is why friction rubs are often best heard in these regions, especially if the lung surfaces are irritated or inflamed, allowing the characteristic sound to be more audible over these expansive areas.

Other options, such as over the diaphragm or along the trachea, do not typically produce a friction rub sound as these areas are either not directly related to pleural movement or are not the primary sites where such sounds are amplified. The lung apices are higher in the thoracic cavity and not necessarily synonymous with the sites of greatest thoracic expansion, making them less likely to be where friction rubs are most effectively heard.