In an adult, % of total body surface required to be involved in a burn injury to cause shock

**Core Concept**
In adult burn injuries, the extent of body surface area (BSA) affected can determine the severity of the injury, including the risk of developing shock. The BSA involved in a burn injury can be estimated using the Lund and Browder chart or the Rule of Nines. Shock in burns is primarily due to the release of inflammatory mediators and the activation of the sympathetic nervous system, leading to vasodilation and decreased peripheral resistance.

**Why the Correct Answer is Right**
The correct answer is 15% to 20% of the total BSA. This is because a burn injury involving 15% to 20% of the BSA can lead to significant fluid loss, electrolyte imbalance, and systemic inflammatory response syndrome (SIRS), ultimately resulting in shock. The release of inflammatory cytokines and the activation of the sympathetic nervous system can cause vasodilation, leading to decreased peripheral resistance and a drop in blood pressure.

**Why Each Wrong Option is Incorrect**
**Option A:** 5% of the BSA is not typically associated with shock, as the body can compensate for the fluid loss and electrolyte imbalance. Burns involving 5% of the BSA are often considered minor and may not require intensive care.

**Option B:** 25% of the BSA is associated with a high risk of shock and multi-organ failure. However, the question asks for the percentage that "causes shock," which implies a more moderate risk, making 25% an overestimation.

**Option D:** 10% of the BSA is below the threshold for shock, and patients with burns involving this percentage may not require aggressive fluid resuscitation.

**Clinical Pearl / High-Yield Fact**
The Parkland formula is used to estimate the fluid resuscitation needs in burn patients: 4 mL/kg/%BSA burned. This formula is essential in managing burn patients and preventing shock.

**Correct Answer: C. 15% to 20%**