Which of the following is a sign of child abuse?

Bruises on the buttocks and retinal hemorrhage are both significant indicators of child abuse. Bruises on the buttocks are often associated with accidental injuries but can be a red flag for physical abuse, especially if they are not consistent with the child's developmental stage or are located in patterns indicative of punishment.

Retinal hemorrhage is a more serious sign and is typically associated with non-accidental trauma, particularly in cases of abusive head trauma, as it may arise from violent shaking or impact. The presence of retinal hemorrhages indicates significant force and raises concerns about potential abusive behavior, especially in very young children who are not independently mobile.

In contrast, bruises on the shins can occur frequently in active children and are more likely to be associated with typical play injuries. Thus, while they can indicate injury, they are not as specific or indicative of abuse compared to bruises on the buttocks and the presence of retinal hemorrhage, which suggest underlying concerns of abuse. This understanding makes "both A and B" the correct choice, as they both pertain directly to potential signs of child abuse.