Which of the following is true of brain activities found in posttraumatic stress disorder (PTSD) sufferers?

A) The amygdala appears to be less responsive to emotional stimuli.
B) The medial prefrontal cortex tends to be unregulated.
C) The hippocampus may shrink due to overexposure to neurotransmitters and hormones.
D) The medial prefrontal cortex is more active in the case of severe symptoms of PTSD.

Studies have shown that the hippocampus, which is responsible for memory, may shrink in individuals with PTSD due to overexposure to stress hormones, such as cortisol. This can lead to difficulties with memory and learning. It is not accurate to say that the amygdala is less responsive or that the medial prefrontal cortex is always unregulated or more active - the research on brain activity in PTSD is complex and not fully understood.