USMLE® Step 1 style questions USMLE
A 2-week-old newborn boy is brought to the emergency department by his parents due to irritability and difficulty feeding over the past few days. Medical history is significant for a red, sac-like lesion noted shortly after birth that was located on the lower back, which presented with flaccid paralysis of the lower limbs and absence of rectal tone. The patient subsequently underwent surgery for the removal of the sac-like lesion. At present, the patient’s temperature is 37.0°C (98.6°F), pulse is 120/min, respirations are 40/min, and blood pressure is 70/40 mmHg. An examination of the eyes is shown below:
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The other eye reveals similar findings. This infant’s underlying condition is most likely caused by which of the following processes?
The cerebellum, or "little brain", is divided into two hemispheres which are separated by a narrow worm-like structure in the middle called the vermis.
Directly beneath the hemispheres are two smaller lobes called cerebellar tonsils.
The cerebellum controls balance, posture, and helps to initiate as well as fine-tune voluntary motor activity - think about the fancy finger work of a piano player or the vocal cords of a singer - that’s the cerebellum in action.
Highest up, are two C-shaped lateral ventricles that lie deep in each cerebral hemisphere.
The lateral spinothalamic tract carries information about pain and temperature, while the anterior spinothalamic tract carries information about touch. These fibers usually decussate, or cross the spinal cord near its center at the anterior white commissure, before finally carrying the signal upwards to the thalamus and brain.
It’s usually caused by underdevelopment of the posterior fossa of the skull during fetal development.
A type I Chiari malformation occurs when only the cerebellar tonsils herniate down into the foramen magnum. As a result, the displaced cerebellum can compress the fourth ventricle and block the normal flow of cerebrospinal fluid from the ventricles into the subarachnoid space.
Over time, cerebrospinal fluid may also buildup in the spinal canal although the exact mechanism of how this occurs remains unknown.
Chiari malformation is a structural defect in the brain which occurs when the cerebellum herniates in the foramen magnum due to craniovertebral junction anomalies. This can cause pressure on the spinal cord and other parts of the brain. There are different types of Chiari malformations, but the common ones are type I and II. Type I Chiari malformation occurs when there is the herniation of only the cerebellar tonsils and is associated with syringomyelia. Type II Chiari malformation (also called Arnold Chiari malformation) occurs when there is herniation of both the cerebellar tonsils and the vermis and is associated with myelomeningocele.
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