Measles is caused by a
**Core Concept**
Measles is a highly contagious viral disease caused by a specific paramyxovirus that primarily affects the respiratory system and the central nervous system. The virus is transmitted through respiratory droplets and can cause severe symptoms, including fever, cough, and a characteristic rash.
**Why the Correct Answer is Right**
The measles virus is a member of the Paramyxoviridae family, which includes other viruses such as mumps and respiratory syncytial virus (RSV). The measles virus infects cells in the respiratory tract and replicates, leading to the production of new viral particles. These particles are then released into the air through coughing and sneezing, allowing the virus to spread to other individuals. The virus also causes an immune response, leading to the characteristic rash and other symptoms associated with measles.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because measles is not caused by a bacterium, such as Streptococcus pyogenes, which causes strep throat.
**Option B:** This option is incorrect because rubella (German measles), while also a viral disease, is caused by a different virus, Rubella virus.
**Option C:** This option is incorrect because varicella (chickenpox), another viral disease, is caused by the Varicella-zoster virus.
**Clinical Pearl / High-Yield Fact**
It's essential to note that measles is a vaccine-preventable disease, and the measles, mumps, and rubella (MMR) vaccine is highly effective in preventing measles infection.
**Correct Answer: D. Measles virus.**