USMLE® Step 1 style questions USMLE
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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?
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.
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.
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 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.
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.
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.