Hematoma: A bruise in the right lower quadrant of the abdomen is most likely due to a hematoma, which is a collection of blood outside of blood vessels, often caused by trauma or injury to the abdominal wall.

Muscle Strain: Strain of the abdominal muscles can cause bruising and pain in the right lower quadrant.
Contusion: A contusion, which is a bruise of the skin and underlying tissues, can occur due to a direct blow to the abdomen.
Intra-abdominal Shunt or Bleeding: Although less common, intra-abdominal bleeding or shunting of blood can cause bruising that appears in the right lower quadrant.

Appendicitis: Although appendicitis typically presents with pain, it can sometimes cause bruising if the appendix ruptures and blood tracks to the abdominal wall.
Intestinal Obstruction: An obstruction in the intestines can lead to ischemia and subsequent bruising of the abdominal wall.
Ectopic Pregnancy: In women, a ruptured ectopic pregnancy can cause severe abdominal pain and bruising.
Ruptured Aneurysm: A ruptured abdominal aortic aneurysm can cause severe abdominal pain and bruising, although this is more common in the midline or left lower quadrant.

Pancreatitis: Pancreatitis can cause bruising of the abdominal wall, known as Grey Turner's sign, although this is more commonly seen in the flanks.
Hemorrhagic Pancreatic Pseudocyst: A rare complication of pancreatitis where a pseudocyst ruptures and causes hemorrhage, potentially leading to bruising in the right lower quadrant.
Abdominal Wall Desmoid Tumor: A rare tumor of the abdominal wall that can cause a mass and potentially bruising if it outgrows its blood supply.