Answers and Explanations

1. Answer: C. The patient has the DiGeorge sequence, which results from improper development of the third and fourth pharyngeal pouches. The thymus and parathyroid glands that develop in these two pouches would be absent; as a result, there would be few T cells in the paracortex of lymph nodes.

2. Answer: B. Multiple sclerosis affects only axons in the CNS that have myelin sheaths formed by oligodendrocytes. The optic nerve is a direct outgrowth of the CNS and is the only nerve that falls into this category. All other nerves are in the PNS and have their myelin sheaths formed by Schwann cells.

3. Answer: D. The parathyroid gland will be missing in this patient.

4. Answer: C. The patient has an infection in the cavernous sinus. The first nerve to be affected would be the abducens nerve resulting in a medially deviated eyeball.

5. Answer: B. Apical lung tumors may compress the lower trunk of the brachial plexus, in particular the Tl ventral ramus. Preganglionic sympathetic axons, which leave the spinal cord in the Tl ventral ramus and synapse in the superior cervical ganglion, provide innervation to the face, scalp, and orbit. A complete lesion of these fibers disrupts sympathetic innervation to the face, scalp, and orbit and results in Horner syndrome.

6. Answer: C. The only muscle among the choices that elevates the mandible is the masseter.

7. Answer: D. Skin of the chin is innervated by branches of the mandibular nerve (V3).

8. Answer: A. The middle meningeal artery is typically lacerated in lateral skull trauma, which results in an epidural hematoma. This blood vessel enters the skull through the foramen spinosum.

9. Answer: E. The tumor has compressed the left vagus nerve just prior to the branch point of the left recurrent laryngeal nerve. The left recurrent laryngeal nerve innervates all of the muscles of die left side of the larynx except for the cricothyroid, resulting in a weakness in the ability to abduct the left vocal cord. The left recurrent laryngeal nerve also innervates mucosa below the vocal fold, which would also be affected. The superior laryngeal nerve innervates the cricothyroid muscle, which tenses the vocal cord, and innervates mucosa above the vocal fold. The vagus nerve does not innervate muscles that act on the hyoid, and vagal branches to pharyngeal muscles used in swallowing are given off in the neck.

10. Answer: B. MaxOlary prominences have failed to fuse with each side of the intermaxillary segment.

] ], Answer: A. The mandible, the malleus, and the incus all are derived from the first pharyngeal arch.

12. Answer: D. The thyroid develops in the midline by utilizing a thyroglossal duct that descends in the midline from the apex of the sulcus terminalis in the tongue.

13. Answer: C. The third aortic arch gives rise to the common carotid arteries.

14. Answer: B. CN IX carries preganglionic parasympathetic axons, which traverse the jugular foramen, synapse in the otic ganglion, and provide secretomotor innervation to the parotid gland.

15. Answer: E. Cut axons result in retrograde changes in the neuronal cell bodies in the nucleus ambiguus.

16. Answer: D. Branches of the Ophthalmic division of V traverse the superior orbital foramen and carry general sensation from the skin of the anterior scalp and dorsum of the nose.

17. Answer: D. The maxillary sinus lies inferior to the orbit.

18. Answer: D. The palatine tonsil develops in the second pharyngeal pouch.

19. Answer: B. The stapes and styloid process are derived from the second pharyngeal arch.

SECTION IV

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