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USMLE® Step 1 questions

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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?  

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Poliovirus p. 549

immunodeficient patients p. 116

medical importance p. 164

picornavirus p. 165

unvaccinated children p. 183


poliovirus p. 183

Sabin poliovirus vaccine p. 164

Vaccines p. 109

Poliovirus p. 164

Poliomyelitis p. 549

restrictive lung disease p. 700

Cerebrospinal fluid (CSF)

poliomyelitis p. 549

Headache p. 536

poliomyelitis p. 549


polio presentation p. 549

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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.


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.


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