USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 771:

  Papez circuit provides the anatomic substrate for the integration of cognitive, emotional, and expressive brain activities. Which of the following brain structures participates in Papez circuit?

  A. amygdale

  B. caudate nucleus

  C. mammilary nuclei

  D. suprachiasmatic nucleus

  E. ventrolateral thalamic nucleus

  Correct Answer: C

  Section: Anatomy Papez circuit is formed by efferent fibers of the hippocampus, which project by way of the fornix to the mammillary nuclei in the hypothalamus. Axons from the mammillary nuclei project to the anterior thalamic nucleus, which is connected to the cingulate gyrus. The latter projects back to the hippocampus by way of the parahippocampal gyrus. Papez circuit integrates cortical, hypothalamic, and thalamic functions resulting in the integration of cognitive, emotional, and expressive brain functions. The ventrolateral thalamic nucleus (choice E) is involved in motor pathways, being connected to the deep cerebellar nuclei and the motor cortex. The caudate nucleus (choice B) is a component of the basal ganglia and the extrapyramidal motor control system. Although the amygdala (choice A) is involved in emotional brain functions, it is not part of Papez circuit. The suprachiasmatic nucleus (choice D) is located in the hypothalamus and mediates the circadian rhythm.

• Question 772:

  An 18-year-old male patient was thrown from a horse and suffered extensive damage to the back and pelvic region. After 2 weeks of recovery, it is noted that the patient has a spastic neurogenic urinary bladder. Although the patient is unable to initiate micturition, the bladder periodically empties itself. Where is the lesion affecting control of the bladder in this patient located?

  A. cauda equine

  B. spinal cord, above the level of S2

  C. spinal cord, at the level of S2

  D. spinal cord, at the level of S3

  E. spinal cord, at the level of S4

  Correct Answer: B

  Section: Anatomy Since the patient presents with a spastic neurogenic bladder, the lesion is in the spinal cord above the level of S2. The micturition reflex works as follows: when the bladder becomes distended, sensory impulses travel back from the bladder to synapse onto preganglionic parasympathetic neurons located at the levels of S2-S4. These neurons, in turn, excite postganglionic parasympathetic neurons which innervate and cause contraction of the detrusor muscle of the urinary bladder, thus emptying the bladder. This primitive reflex loop comes under supraspinal influence in early childhood when the child learns control of the micturition reflex. When the supraspinal influence is removed, as happens with spinal lesions above the level of S2, the primitive reflex loop remains and the neurogenic spastic bladder empties itself reflexively whenever it becomes full. Lesions in the cauda equina (choice A), or in the spinal cord at the level of S2 (choice B), S3 (choice D), or S4 (choice E) will result in a flaccid bladder because of loss of innervation to the detrusor muscle. In this case, the bladder does not contract by itself and the patient will have to be catherized to prevent cystitis.

• Question 773:

  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?

  A. ascends the brainstem in the medial lemniscus

  B. decussates in the ventral white commissure of the spinal cord

  C. has its cell body in the nucleus gracilis

  D. is found in the dorsal funiculus of the spinal cord

  E. terminates in the nucleus cuneatus

  Correct Answer: A

  Section: Anatomy 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).

• Question 774:

  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?

  A. in the territory of the left anterior cerebral artery

  B. in the territory of the left middle cerebral artery

  C. in the territory of the left posterior cerebral artery

  D. in the territory of the right middle cerebral artery

  E. in the territory of the right posterior cerebral artery

  Correct Answer: B

  Section: Anatomy 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.

• Question 775:

  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?

  A. fourth cranial (trochlear) nerve

  B. medial longitudinal fasciculus (MLF)

  C. second cranial (optic) nerve

  D. sixth cranial (abducens) nerve

  E. third cranial (oculomotor) nerve

  Correct Answer: B

  Section: Anatomy 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.

• Question 776:

  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?

  A. fifth

  B. first

  C. fourth

  D. second

  E. third

  Correct Answer: E

  Section: Anatomy 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).

• Question 777:

  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?

  A. buccinators

  B. levator labii superioris

  C. levator labii superioris alaeque nasi

  D. levator palpebrae superioris

  E. orbicularis oculi

  Correct Answer: D

  Section: Anatomy 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.

• Question 778:

  Desmin is a 53-kDa protein found in skeletal and smooth muscle cells. It plays a role in coordinating muscle cell contraction. Desmin belongs to which type of intermediate filaments?

  A. type I

  B. type III

  C. type IV

  D. type V

  E. type VI

  Correct Answer: B

  Section: Anatomy Desmin belongs to the type III intermediate filaments. Intermediate filament proteins have three domains: a head domain (N-terminal), an alpha-helical rod domain, and a tail domain (C-terminal). The six types of intermediate filament proteins are differentiated by the sequence in the rod domain. Type I (choice A) intermediate filaments include the cytokeratins. Type IV (choice C) intermediate filaments are found in the nervous system and include the neurofilaments and alpha-internexin. Type V (choice D) intermediate filaments are the nuclear lamins. Nestin, expressed in stem cells of the central nervous system, is a type VI (choice E) intermediate filament.

• Question 779:

  Which of the following normally occupy the dark spaces indicated by the arrows in following figure?

  A. blood vessels

  B. differentiating blood cell precursors

  C. osteoblasts

  D. osteoclasts

  E. osteocytes

  Correct Answer: E

  Section: Anatomy The black areas in this section of dried, compact bone are empty spaces normally occupied by cells and soft tissues. The arrows indicate lacunae interspersed among lamellae of osteons (haversian systems). In life, these are occupied by osteocytes. The fine lines radiating from the lacunae are canaliculi, which contain processes of the osteocytes. Blood vessels (choice A) of compact bone course through Volkmann's canals (not shown) and haversian canals (seen here at the center of each osteon). Hematopoietic cells (choice B) occur in red marrow located within medullary canals of long bones and the cavities of cancellous bone. Osteoblasts (choice C) are restricted to surfaces of bone at sites of bone apposition. When osteoblasts become entrapped in lacunae as a result of their synthetic activity, they become osteocytes. Osteoclasts (choice D) are large, multinucleated cells found at surfaces of bone at sites that are undergoing absorption.

• Question 780:

  An overworked surgical resident is resecting an acinic cell carcinoma from the inferior aspect of the parotid gland when he makes a careless incision and nicks a branch of the facial (seventh cranial) nerve. Which of the following muscles may be affected by this injury?

  A. omohyoid

  B. sternohyoid

  C. sternothyroid

  D. stylohyoid

  E. thyrohyoid

  Correct Answer: D

  Section: Anatomy The stylohyoid is innervated by the facial (seventh cranial) nerve. During the embryonic period, the muscles developing from the second pharyngeal arch include the stylohyoid, stapedius, posterior belly of the digastric, auricular muscles, and muscles of facial expression. The nerve supplying the second pharyngeal arch, and thus all these muscles, is the facial (seventh cranial) nerve. All the other muscles, omohyoid (choice A), sternohyoid (choice B), sternothyroid (choice C), and thyrohyoid (choice E) form the infrahyoid muscle. They are supplied as a group by branches of the ansa cervicalis derived from C1, C2, and C3.