In coding burns, ICD-10-CM guidelines indicate that each burn site is coded separately, specifying the degree, location, and extent of the burn. Additionally, a code for total body surface area (TBSA) burned is included when burns cover more than 10% of the body. Here’s the breakdown:

1. T24.311A: Represents a third-degree burn on the left thigh, initial encounter.

2. T24.312A: Represents a third-degree burn on the right thigh, initial encounter.

3. T21.21XA: Represents a second-degree burn on the anterior chest wall, initial encounter.

4. T21.23XA: Represents a second-degree burn on the upper back, initial encounter.

5. T31.21: Represents burns with 20–29% TBSA involvement, with third-degree burns covering 10–19% of the TBSA.

Explanation of incorrect answers:

A includes an incorrect TBSA code (T31.21).

B has the correct codes but lists an incorrect TBSA code for third-degree burns.

C uses incorrect burn site codes for the areas involved and incorrect TBSA percentages.

Therefore, the correct answer is D. T24.311A, T24.312A, T21.21XA, T21.23XA, T31.21, which accurately captures the burns' degrees, locations, and TBSA.