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USMLE STEP1 Exam (page: 15)
USMLE Step1
Updated on: 15-Feb-2026

Viewing Page 15 of 171
STEP1 PDF 845 Questions

A 19-year-old camper fell asleep with the right side of her face against the cold earth on an autumn night. In the morning, she woke up and found that she could not move the right side of the face. She went to the local emergency room in a nearby town and was diagnosed with Bell's palsy. Which of the following muscles is not affected in her condition?

  1. buccinators
  2. levator labii superioris
  3. levator labii superioris alaeque nasi
  4. levator palpebrae superioris
  5. orbicularis oculi

Answer(s): D

Explanation:

The levator palpebrae superioris, which opens the upper eyelid and is located in the orbit, is innervated by the oculomotor (third cranial) nerve. In this case of Bell's palsy, longterm exposure to the cold has affected the peripheral portion of the facial (seventh cranial) nerve. The buccinator (choice A), levator labii superioris (choice B), levator labii superioris alaeque nasi (choice C), and orbicularis oculi (choice E) are all innervated by the facial nerve and will all be impaired.



In examining a 30-year-old male patient with multiple sclerosis, you notice that when you ask the patient to look to the left, the right eye fails to adduct and when the patient looks to the right, the left eye fails to adduct. However, both eyes in the patient can adduct during convergence. There is no noticeable strabismus when the eyes are focused on a far point. During abduction, both eyes display nystagmus. You record on the patient's chart a diagnosis of bilateral internuclear opthalmoplegia.
Which of the following neural structures is affected?

  1. fourth cranial (trochlear) nerve
  2. medial longitudinal fasciculus (MLF)
  3. second cranial (optic) nerve
  4. sixth cranial (abducens) nerve
  5. third cranial (oculomotor) nerve

Answer(s): B

Explanation:

Conjugate movement of the eyes depends on the integrity of the MLF, which links the ipsilateral lateral gaze center in the pons with the contralateral oculomotor nucleus in the midbrain. Lesion of the MLF disconnects these brain nuclei resulting in internuclear ophthalmoplegia and the bilateral condition can be seen in patients with multiple sclerosis. A patient with a lesion of the fourth cranial nerve (choice A) cannot look downward and inward; there is a slight convergent strabismus when the patient attempts to do so.
Lesion of the second cranial nerve (choice C) will result in blindness. Weakness of eye abduction characterizes lesion of the sixth cranial nerve (choice D), which also displays s a convergent strabismus. In a patient with a third cranial nerve (choice E) lesion, the eye is deviated downward and outward (divergent strabismus); ptosis would also be noticeable.



You are examining a 12-year-old male patient who has a slowly enlarging, painless swelling in the left inferior region of the neck. After careful palpation and consideration of the results of the radioimaging studies, you diagnose a branchial cyst in the left inferior parathyroid gland. This gland arose in development from which of the following pharyngeal pouches?

  1. fifth
  2. first
  3. fourth
  4. second
  5. third

Answer(s): E

Explanation:

The inferior parathyroid glands and the thymus arise from the third pharyngeal pouch. The superior parathyroid glands develop from the fourth pharyngeal pouch (choice C). The fifth pharyngeal pouch (choice A) in the human is rudimentary or absent. The first pharyngeal pouch (choice B) gives rise to the tympanic membrane, tympanic cavity, mastoid antrum, and pharyngotympanic tube. The palatine tonsil along with the tonsillar sinus and crypts develop from the second pharyngeal pouch (choice D).



Regarding the axon of the second-order neuron in the pathway for conscious awareness of fine, discriminative touch and vibratory sensation from the upper limb, which of the following is correct?

  1. ascends the brainstem in the medial lemniscus
  2. decussates in the ventral white commissure of the spinal cord
  3. has its cell body in the nucleus gracilis
  4. is found in the dorsal funiculus of the spinal cord
  5. terminates in the nucleus cuneatus

Answer(s): A

Explanation:

The sensations of discriminative touch and vibration are transmitted through the medial lemniscus. Pain and temperature pathways decussate in the ventral white commisssure (choice B). The nucleus gracilis (choice C) contains neurons that process sensory signals from the lower extremity. The second-order fibers carrying discriminative touch and vibration from the upper limb originate from neurons in the nucleus cuneatus (choice E). First order fibers are found in the dorsal funiculus (choice D).



A 60-year-old male patient is brought to the hospital following sudden onset of weakness and sensory loss in the right face and upper limb. The right lower limb is unaffected. An MRI scan would reveal signs of a stroke in which of the following areas?

  1. in the territory of the left anterior cerebral artery
  2. in the territory of the left middle cerebral artery
  3. in the territory of the left posterior cerebral artery
  4. in the territory of the right middle cerebral artery
  5. in the territory of the right posterior cerebral artery

Answer(s): B

Explanation:

Because the right side of the patient is affected, the stroke is in the territory of the left middle cerebral artery. This artery supplies the lateral aspect of the cerebral hemisphere, including portions of the pre- and postcentral gyri corresponding to the head, upper limb, and trunk on the primary motor (area 4 of Brodmann) and primary sensory (area 3, 1, 2 of Brodmann) cortical strips. These cortical control areas for the right lower limb are supplied by branches of the left anterior cerebral rtery (choice A), which is uninvolved in this case since the lower limb is intact. The left posterior cerebral artery (choice C) supplies the occipital and temporal lobes and is unaffected in this case. Since the right side of the brain controls the left side of the body and the patient is intact on the left side, none of the right side cerebral arteries (choices D and E) are involved.



Viewing Page 15 of 171



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