Final answer:
A. Genetic mutation
The deterioration in 'Little Girl Lost: A Case Study on Defective Cellular Organelles' was triggered by a genetic mutation affecting the mitochondria.
Step-by-step explanation:
The event that triggered Nicole’s deterioration was A. Genetic mutation. Inherited mitochondrial diseases, such as the one affecting Nicole, can be caused by damage to the DNA within the mitochondria. Such genetic defects are not typically induced by environmental factors, viral infections, or lifestyle choices. Instead, they are the result of mutations that can impact the function of cellular organelles like mitochondria. These mutations can cause a variety of symptoms, which can present differently within members of the same family due to the number of affected mitochondria in each cell. As the mitochondrial DNA gets replicated during cell division, the daughter cells could inherit varying amounts of this damaged DNA, leading to a diversity in symptoms among affected individuals.