## **Core Concept**
Primary blast injuries result from the direct effects of blast wave (shockwave) propagation through tissues. These injuries are caused by the rapid changes in air pressure that occur during an explosion. The organs most susceptible to damage are those with air-filled spaces or significant pressure differentials.

## **Why the Correct Answer is Right**
The lungs are the commonest organ to be injured in primary blast injuries because they have air-filled spaces and are highly compliant, making them more susceptible to barotrauma. The blast wave can cause alveolar rupture, pneumothorax, and hemorrhage. The lung's large surface area and the presence of air-liquid interfaces make it particularly vulnerable to the effects of the pressure wave generated by an explosion.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While the middle ear can be injured due to the pressure changes caused by a blast, it is not the most common organ injured in primary blast injuries. Middle ear injuries are more related to the effects of the blast wave on air-filled spaces but are generally less severe and less frequently life-threatening compared to lung injuries.
- **Option B:** The gastrointestinal tract can also be injured in primary blast injuries, particularly due to the transmission of the blast wave through fluid-filled structures. However, it is less commonly injured compared to the lungs.
- **Option C:** This option seems to be missing but based on the context, if it were another organ, it would be incorrect for the reasons specified for options A and B.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that primary blast injuries often affect air-filled organs like the lungs and middle ear. Clinicians should have a high index of suspicion for lung injuries, including pneumothorax and alveolar damage, in patients exposed to blasts.

## **Correct Answer:** . Lung