Final answer:
Genetic predisposition refers to the likelihood of developing PTSD due to one's genes, which, when combined with environmental stressors, can lead to the onset of PTSD.
Step-by-step explanation:
The term that explains how stress can trigger PTSD in people with high-risk genes is a) Genetic predisposition. This term refers to the increased likelihood of developing a particular disease based on a person's genetic makeup. In the context of PTSD, certain individuals may have genes that make them more susceptible to stressors, leading to a higher risk of developing PTSD when exposed to traumatic events. These genetic vulnerabilities, in combination with environmental factors such as exposure to military combat, physical assaults, or natural disasters, can interact to influence the onset of PTSD. This interaction between genetics and environment is an aspect of the broader field of study known as epigenetics. Epigenetic changes can be triggered by exposure to various stressors and can have long-term effects on gene expression and health outcomes.