USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 7-month-old infant boy is brought to the emergency department by his father because of a 2-day history of vomiting and watery diarrhea. His temperature is 38.4°C (101.1°F), pulse is 133/min, respirations are 34/min, and blood pressure is 101/60 mm Hg. Physical examination shows dry mucous membranes and decreased skin turgor. Laboratory studies show:
ELISA is performed on a stool sample which tests positive for a virus which is known to have 11 double-stranded molecules of RNA. Which of the following is the most likely causal organism?
In fact, it’s believed that almost every child has had the vaccine or been infected by this virus at least once before the age of five.
There are at least eight strains of rotaviruses, named A to H, but most rotavirus gastroenteritis infections in humans are due to group A strains.
Rotavirus is part of the reovirus family, where reo- stands for respiratory, enteric, and orphan.
This tells us that this family of viruses affect either the respiratory system, the intestines, or they’re “orphans” who don’t cause any disease, as far as we can tell.
They are surrounded by a double icosahedral capsid, which is a two-layered spherical protein shell made up of 20 equilateral triangular faces.
And they’re “naked” because the capsid isn’t covered by a lipid membrane.
They’re RNA viruses with double stranded linear RNA with 10 to 12 segments.
This means that they must first transcribe their genetic material into single stranded mRNA before host cell ribosomes can use it to make viral proteins.
Transcription happens inside the capsid, using a viral enzyme called RNA-dependent RNA polymerase.
Then the mRNA enters the host cell’s cytoplasm where ribosomes are found.
Rotavirus is transmitted from person to person via the fecal-oral route.
In other words, you catch it by ingesting the stool or vomit particles of someone who is sick. Yuck.
This can happen if infected stool ends up in the water supply or on agricultural fields, if flies land on it, and transfers stool particles to other places, or by touching contaminated surfaces.
You can summarize it as the four Fs: fluids, fields, flies, and fingers.
As a result, rotavirus can end up in food and drinking water - and from there, it makes its way to the small intestine.
Now, the small intestine has lots of tiny ridges and grooves, each of which projects little finger-like fibers called villi.
And in turn, each villus is covered in teeny tiny little microvilli. This is called the brush border.
All of this gives the small intestines plenty of surface area to absorb nutrients.
The brush border is also where many digestive enzymes are produced by intestinal epithelial cells, or enterocytes.
Rotavirus lowers the intestine’s ability to absorb nutrients by decreasing activity of digestive enzymes, like maltase and lactase, and by producing a viral protein called non-structural protein 4.
Non-structural protein 4 damages villi by inducing apoptosis, or cell death, of mature enterocytes, which are replaced by immature enterocytes that are less effective absorbers.
Rotavirus also activates the enteric nervous system, which is the division of the autonomic nervous system that helps regulate the activity of the GI tract, and this triggers secretion of electrolytes, like sodium and potassium ions, and water into the intestinal lumen.
Ok, now rotavirus is very contagious, so you only need to come in contact with a small amount to catch it.