Final answer:
Periumbilical bruising (Cullen's sign) is the best indicator of internal abdominal bleeding in passengers involved in a collision, whereas bruising along the flank (Grey Turner's sign) may suggest retroperitoneal hemorrhage.
Step-by-step explanation:
The best indicator of internal abdominal bleeding in a group of bus passengers involved in a collision being triaged is periumbilical bruising (A). This type of bruising, often referred to as Cullen's sign, occurs when blood accumulates under the layer of skin that surrounds the belly button, and it is a significant indicator of intra-abdominal bleeding, possible hemorrhagic pancreatitis, or ruptured ectopic pregnancy. It is crucial for emergency medical personnel to recognize these signs to prioritize care and quickly attend to potential life-threatening injuries.
Bruising along the right or left flank, known as Grey Turner's sign, could also indicate retroperitoneal hemorrhage, usually associated with pancreatitis or other internal injuries.
However, bruising in an oblique pattern across the chest (C) or bruising of the inguinal areas (D) are less specific for abdominal bleeding and could be related to other types of injuries.