**Core Concept**
Pus formation in burns is a result of the body's immune response to bacterial invasion, characterized by the accumulation of leukocytes, bacteria, and cellular debris in the burn wound. This process is influenced by factors such as the depth and size of the burn, as well as the presence of infection.

**Why the Correct Answer is Right**
Pus typically forms within 2-3 days after a burn injury due to the rapid onset of bacterial colonization and the subsequent inflammatory response. The burn eschar (dead tissue) provides an ideal environment for bacterial growth, leading to the activation of neutrophils and macrophages, which then accumulate in the wound to fight the infection. The formation of pus is a sign of the body's attempt to contain and eliminate the infection.

**Why Each Wrong Option is Incorrect**
**Option B:** This option is incorrect because pus formation typically occurs within the first 2-3 days after a burn injury, not 3-5 days. While it's possible for pus to form later, this timeframe is generally too late for initial pus formation.
**Option C:** This option is incorrect because pus formation within 2-3 weeks is highly unlikely in the context of burns. By this time, the wound would have already progressed to more severe stages, such as gangrene or sepsis.
**Option D:** This option is incorrect because pus formation within 4 weeks is extremely rare in burn injuries. By this time, the wound would have already been treated and managed, and pus formation would be an indication of a severe complication.

**Clinical Pearl / High-Yield Fact**
It's essential to note that early recognition and management of burn infections are critical to prevent severe complications. Pus formation in burns often indicates a need for prompt debridement (removal of dead tissue) and antibiotic therapy to prevent sepsis.

**✓ Correct Answer: A. 2-3 Days**