What is the most likely outcome for a person exposed to measles who has not previously had chicken pox or been vaccinated?

A person who has been exposed to measles and has not previously had chickenpox or been vaccinated is most likely to develop measles illness. Measles is highly contagious, and the virus can be transmitted through respiratory droplets from an infected person. The absence of prior immunity—either from an infection with measles or through vaccination—means that the individual's immune system has not yet encountered the virus and therefore does not have pre-existing antibodies or an immune response to combat the infection.

When exposed to the measles virus, the individual’s immune system will respond by trying to detect and fight the new pathogen. However, without previous exposure or vaccination, this response is not immediate and often the virus can replicate enough to cause symptomatic illness. Symptoms of measles typically include fever, cough, runny nose, and a characteristic rash, all stemming from the body's attempt to handle the acute infection.

While options suggesting an immune response or pathways involving existing antibodies may seem valid, they do not apply in this case since the individual has no prior immunity to either measles or chickenpox. Hence, developing measles disease is the most likely outcome in this scenario.