**Core Concept:** Blast injury is a type of traumatic injury caused by the sudden expansion of gas, typically in military conflicts or terrorist attacks, leading to a variety of organ injuries. The blast wave, a high-pressure and high-speed airflow, affects the body's tissues and organs differently based on their size, shape, and distance from the blast source.

**Why the Correct Answer is Right:** The least vulnerable organ to the blast wave is the brain. The human head is relatively small and has a rounded shape, which helps to distribute the pressure forces evenly across the skull. This reduces the risk of direct mechanical trauma to the brain.

**Why Each Wrong Option is Incorrect:**
A. Heart (Myocardium): The heart is not as vulnerable as the brain due to its size and shape, but the rapid acceleration and deceleration caused by the blast wave can lead to myocardial injury.
B. Liver: The liver is larger than the brain, and its flat surface increases the risk of direct trauma and rupture due to the blast wave's impact.
C. Lungs: The lungs are easily ruptured due to their thin walls, but the blast wave's rapid pressure changes can lead to pneumothorax and pulmonary contusions.
D. Kidneys: The kidneys are also vulnerable to direct trauma due to their size, shape, and position within the ribcage, increasing the risk of blunt injuries.

**Clinical Pearl:** The least vulnerable organ to blast injury is the brain because of its size, shape, and the distance from the blast source. Most blast injuries occur due to the secondary effects of the blast wave, such as rapid acceleration/deceleration and the so-called "triple-tap" mechanism involving three phases of injury: the primary blast injury, the secondary injury, and the tertiary injury. Rapid deceleration, shearing forces, and impact on surrounding tissues are more likely to cause injury to larger organs like the heart, lungs, liver, and kidneys.