Two individuals, Sarah and Maria, are involved in the same severe car accident. Both experience significant distress in the immediate aftermath. Genetic screening reveals that Sarah has a common variation in a gene that regulates serotonin, which is associated with an increased likelihood of developing certain stress-related disorders. Maria does not have this genetic variation. Six months later, Sarah is diagnosed with Post-Traumatic Stress Disorder (PTSD), while Maria has recovered well and does not meet the diagnostic criteria. Analyze this scenario and explain the most likely biological reason for the difference in their long-term outcomes.