Answers and Explanations

1. Answer: C. Oligodendrocytes form myelin for all myelinated axons inside the CNS, including tracts in the white matter of the spinal cord.

2. Answer: E. The patient has a combination of upper and lower motor neurons signs characteristic of ALS. In this case, the ALS has affected the cervical enlargement first resulting in lower motor neurons signs in the upper limbs and upper motor signs in the lower limbs.

3. Answer: E. A ventral root contains axons of lower motor neurons and is found in the peripheral nervous system. Here all myelin is formed by Schwann cells, which promote regeneration of cut axons. Choices A, B, and C are signs attributable to upper motor neuron disease, and central and peripheral processes of dorsal root ganglion cells course in dorsal roots and would not be affected.

4. Answer: B. A hemisection of the spinal cord (Brown Sequard syndrome) produces an ipsilateral paresthesia below the lesion, in this case, below the T10 dermatome including the lower limb. Pain and temperature would be lost only in the lower limb on the left, hyperactive stretch reflexes would be seen in the right limb, absent stretch reflexes would be seen only at the level of the lesion and could not be demonstrated in this case, and Horner syndrome might seen in hemisections in the cervical cord.

5. Answer: A. The patient presents with the three "Ps" of tabes dorsalis; pain, paresthesia, and polyuria, characteristic of tabes dorsalis and caused by late-stage neurosyphilis. These patients may also present with Argyll Robertson pupils, which accommodate but do not constrict in response to light.

6. Answer B. The optic nerves are the only nerves that have myelin formed by oligodendrocytes that degenerate in multiple sclerosis. MS is indicated by the neurological deficits separated by space and time and the oligoclonal banding. All of the other choices indicate deficits seen in lesions of cranial or spinal nerves, which have myelin formed by Schwann cells.

7. Answer: D. The patient has a bilateral loss of pain and temperature sensations in the hands at the level of the lesion and bilateral lower motor neuron weakness in the hand also at the level of the lesion, indicative of a syrinx at the level of the cervical enlargement of the cord. The bilateral pain and temperature loss is seen first, and as the syrinx expands, the lower motor neurons to muscles in the same regions are affected.

8. Answer: B. A lesion of the second-order crossed axons in the medial lemniscus on the right is the only choice that would result in a loss of two-point discrimination in the left hand.

9. Answer: D. The fasciculus cuneatus begins at about the T5 segment of the spinal cord.

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