USMLE® Step 1 style questions USMLE
A 14-week-old girl is brought to the pediatrician for a follow-up appointment. Her parents are concerned since their neighbor’s child recently became ill and developed asymmetric weakness of the lower extremities. The patient was born in Pakistan, and her birth history was uncomplicated. During the visit, the patient is given oral droplets to prevent infection against an RNA enterovirus. This pharmaceutical agent works via which of the following mechanisms of action?
Content Reviewers:Rishi Desai, MD, MPH, Yifan Xiao, MD
Contributors:Tanner Marshall, MS, Gil McIntire
In poliomyelitis, also called polio, “polio” refers to the poliovirus, which is an enterovirus that invades the intestines, “myel” refers to the spinal cord which is affected in the disease, and -itis refers to inflammation.
So poliomyelitis is an enteroviral disease first enters the body through the intestines, but then spreads and causes nerve injury in the spinal cord.
Former US president Franklin D. Roosevelt contracted polio when he was a baby, and it left him wheelchair-bound.
Broadly speaking, the nervous system consists of two parts.
The central nervous system consists of the brain and the spinal cord.
So the peripheral nervous system includes the nerves that fan out from the central nervous system to reach the skin, muscles, and organs.
Now looking at the cross-section of the brain, there’s gray matter at the periphery of the brain. This is called the cerebral cortex and it consists of nerve cell bodies.
Just inside the gray matter of the brain, is the white matter, and it consists of nerve axons.
In contrast, if you look at the cross-section of the spinal cord, the white matter is on the outside and the gray matter is on the inside, and overall it kinda looks like a butterfly.
If we draw a horizontal line through the spinal cord, the front half is the anterior or ventral half, and the back half is the posterior or dorsal half.
And the butterfly wings are sometimes referred to as horns; so we have two dorsal horns that contain cell bodies for sensory neurons and two ventral horns that contain cell bodies for motor neurons.
So for example, if you step on a lego in your living room, the sensation of discomfort is carried from the nerves in your foot, through the peripheral nervous system to reach the dorsal horn in the spinal cord.
It then travels up the spinal cord to the brain, letting you know that there’s tissue damage.
In response, your brain sends a message through the upper motor neurons, which are part of the cerebral cortex, and down the spinal cord to a lower motor neuron which is located in the anterior horn of the spinal cord.
From there, the signal finally gets delivered to the leg muscles and allows you to lift your foot.
In addition to sending signals from the brain, these lower motor neurons also release trophic or growth factors that promote muscle growth in the muscle that they innervate.
Polio is a viral infection caused by poliovirus which is a single-stranded RNA virus that’s protected by a capsid, which is a protein coat.
Polio mainly affects children under the age of 5 and it’s spread by fecal-oral transmission, which means that the virus usually enters the body through contaminated food and water that goes in the mouth.
It’s also transmitted when an infected person sneezes or coughs, which spreads thousands of virus-containing droplets into the local area.
Once the virus enters the body, it binds to mucosal cells of the oropharynx and small intestine, and gets inside those cells, and then releases its RNA.
The viral RNA uses the cell’s RNA polymerase enzyme to make copies of itself, and then the new RNA copies hijack the ribosomes that normally make proteins for the cell, and forces the ribosomes to make viral proteins instead.
The viral protein and RNA self-assemble into lots of new polioviruses, and within days, they cause the mucosal cell to lyse, releasing the newly formed viruses which make their way to nearby lymph nodes and eventually into the bloodstream.
Poliovirus prefers to infect motor nerves, so oftentimes it will leave a blood vessel and get into the interstitial tissue of muscle tissue.
From there, poliovirus invades the motor neuron and travels retrograde - meaning backwards up through the axon - to the anterior horn of the spinal cord.
Infected motor neurons attract immune cells like neutrophils and macrophages which cause inflammation and damage to the spinal cord.
As infected motor neurons die, the muscles of the trunk and limbs no longer receive signals from the brain or trophic factors which causes the muscles to start to atrophy and become weak.
On rare occasions, poliovirus can cause bulbar polio, which is when it affects a part of the brain stem that has motor nerves involved in speaking and swallowing.
This part of the brain stem also sends motor nerves to the diaphragm and so if they get damaged, it can impair breathing.
Poliovirus is a single-stranded RNA virus, known to cause poliomyelitis. It is transmitted through fecal-oral transmission or by aerosol droplets, and then replicates in the small intestine and oropharynx before it spreads to the central nervous system and causes nerve injury. Symptoms include high fevers, intense muscle pain from spasms and weakness, loss of muscle reflexes, and eventually paralysis. Diagnosis is done by the detection of the poliovirus from a stool or throat, or by analysis of the cerebrospinal fluid. There is no cure for polio, but it can be prevented by polio vaccines.