**Core Concept**
Abdominal wall hematomas occur due to bleeding from the blood vessels in the abdominal wall muscles. The most commonly involved muscle is usually one that is subjected to high stress and strain, such as the external oblique muscle.

**Why the Correct Answer is Right**
The external oblique muscle is the most commonly involved muscle in cases of abdominal wall hematoma due to its location and function. It is the most superficial muscle of the abdominal wall and is subjected to significant stress during activities that involve heavy lifting or sudden movements. The external oblique muscle also has a rich blood supply, which can contribute to the risk of bleeding in the event of trauma or injury.

**Why Each Wrong Option is Incorrect**
* **Option A:** The internal oblique muscle is also a part of the abdominal wall, but it is not the most commonly involved muscle in cases of abdominal wall hematoma. This is because it is deeper and less superficial than the external oblique muscle.
* **Option B:** The transversus abdominis muscle is the deepest muscle of the abdominal wall and is not typically involved in cases of abdominal wall hematoma. This is because it is less superficial and less subjected to stress and strain.
* **Option C:** The rectus abdominis muscle can be involved in cases of abdominal wall hematoma, particularly in the setting of a rectus sheath hematoma. However, this is less common than external oblique muscle involvement.

**Clinical Pearl / High-Yield Fact**
It is essential to consider the clinical context and mechanism of injury when evaluating a patient with an abdominal wall hematoma. A thorough physical examination and imaging studies, such as ultrasound or CT scan, can help identify the involved muscle and guide management.

**Correct Answer: C. Rectus abdominis muscle is sometimes involved, but external oblique muscle is the most common.