USMLE® Step 1 style questions USMLE
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A 6-month-old boy was brought to the emergency department by his parent because of a high-grade fever and a rash for the past 2 days. The rash first appeared on his face and has now spread over his entire body. Prior to the onset of the rash, the patient had a cough, rhinorrhea and pink eyes. He has not received any vaccinations due to parental preference. The rest of the history is noncontributory. Temperature is 39.6°C (103.3°F), pulse is 98/min, respirations are 31/min and blood pressure is 85/60 mmHg. Physical examination shows a tired and restless child with a diffuse, erythematous, maculopapular rash all over the body sparing the palms and soles. Oral examination shows tiny, white round lesions on the buccal mucosa. No pharyngeal exudates are seen. The patient is at increased risk of developing which of the following complications?
Content Reviewers:Rishi Desai, MD, MPH
The reason why this guy’s so contagious is that it’s airborne, and spreads via tiny liquid particles that get flung into the air when someone sneezes or coughs, and can live for up to two hours in that airspace or nearby surfaces.
If someone breathes in that air or touches a surface and then touches their eyes, their eyes, or their mouths, they can become infected.
Measles is so contagious that if one person has it, 90% of nearby non-immune people will also become infected.
Measles virus uses a protein on its surface called hemagglutinin, or just H protein, to bind to a target receptor on the host cell, which could be CD46, which is expressed on all nucleated human cells, CD150, aka signaling lymphocyte activation molecule or SLAM, which is found on immune cells like B or T cells, and antigen-presenting cells, or nectin-4, a cellular adhesion molecule.
Once bound, the fusion, or F protein helps the virus fuse with the membrane and ultimately get inside the cell.
Now this virus is a single-stranded RNA virus, and it’s also a negative sense, meaning it first has to be transcribed by RNA polymerase into a positive-sense mRNA strand.
After that it’s ready to be translated into viral proteins, wrapped in the cell’s lipid envelope, and sent out of the cell as a newly made virus.
From there it continues to spread, eventually getting into the blood and spreading to more lung tissue, as well as other organs like the intestines and the brain.
Now it typically takes 10-14 days from the time that the virus entering the body to the beginning of symptoms, and this is the incubation period.
Once symptoms start, we’ve entered the prodromal period which typically lasts around 3 days, and starts with a high fever and the 3 C’s —cough, conjunctivitis—or inflammation and redness of the white part of the eye, and coryza—swelling in the mucous membrane of the nose, essentially a stuffy nose.
1-2 days later comes the enanthem which is a rash on the mucus membranes, that looks like salt grains on a wet background.
After these initial prodromal symptoms comes the exanthem phase, which is where a red, blotchy, maculopapular rash spreads in a cephalocaudal progression.