Final answer:
In genetics, penetrance refers to the expression of a genotype in the phenotype, while prevalence is the term for the existing number of cases of a disease at a point in time. The correct answer to the student's question is D. Prevalence.
Step-by-step explanation:
Prevalence is the number, or proportion, of individuals with a particular illness in a given population at a point in time. Incidence, on the other hand, is the number or proportion of new cases in a period. For example, if we consider the prevalence of a specific infectious disease, it refers to the percentage of people in a specific age group who are infected with it at a particular time. On the other hand, incidence would refer to the number of new cases of that disease within the same period.
So, in the context of the question, the number of existing cases of mental illness at a point in time would be called prevalence (option D).
In biology, specifically in the context of genetics and epidemiology, the term penetrance refers to the likelihood that a person carrying a particular genotype will actually express the phenotype, or characteristics, associated with that genotype, which can include hereditary diseases.
The term for the number of existing cases of a particular disease at a point in time within a given population is called prevalence. On the other hand, incidence refers to the number of new cases of a disease that occur in a specified period.
So, the correct answer to the question is D. Prevalence. This term is particularly important as it helps estimate the likelihood that someone in a group will have a disease, and it is commonly used to gauge the overall healthcare burden of a disease.