**Core Concept**
Primary blast injury occurs due to the rapid increase in pressure from an explosion, causing damage to the body through the transfer of energy. The effects of primary blast injury are most pronounced on hollow organs filled with gas, such as the lungs and gastrointestinal tract.

**Why the Correct Answer is Right**
The most common organ affected in primary blast injury is the lung. This is because the lungs are filled with air, making them highly susceptible to the rapid changes in pressure caused by a blast. The alveoli in the lungs are damaged due to the over-expansion and subsequent collapse, leading to respiratory distress.

**Why Each Wrong Option is Incorrect**
**Option A:** Liver - While the liver can be affected by secondary and tertiary blast injuries, it is not the most common organ affected by primary blast injury.
**Option B:** Kidney - The kidneys are also susceptible to secondary and tertiary blast injuries, but are not typically affected by primary blast injury.
**Option C:** Brain - Primary blast injury can cause concussive effects on the brain, but it is not the most common organ affected.

**Clinical Pearl / High-Yield Fact**
Primary blast injury can occur even without direct contact with the explosive source, making it a significant concern for individuals in proximity to a blast.

**Correct Answer: A. Lung.