USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 791:

  A 24-year-old student is brought to the emergency room after being found in a ditch where he had lain overnight after being hit by a car. He complains of severe pain in the left arm and examination reveals a broken humerus. Neurological examination reveals that the patient can extend the elbow but displays inability to supinate the elbow when it is extended. The patient also has wrist drop and very weak hand grasp. The neurological lesion is likely localized at which of the following locations? In the above patient, when the elbow is partially flexed, the patient can supinate the left forearm. This is due to the function of which of the following?

  A. biceps brachii

  B. brachialis muscle

  C. brachioradialis

  D. pronator teres

  E. anconeus

  Correct Answer: A

  Section: Anatomy The biceps brachii is a strong supinator of the forearm through its attachment to the radial tuberosity, but it can only function when the elbow is partially flexed. The supinator by itself is a weaker supinator muscle: prove it to yourself by using a screwdriver with your right elbow flexed (biceps brachii and supinator) and your right elbow extended (supinator only). The brachialis (choice B) and brachioradialis (choice C) muscles are flexors of the forearm. The pronator teres (choice D), as its name indicates, pronates the forearm. The anconeus (choice E) helps in extension of the elbow.

• Question 792:

  A 24-year-old student is brought to the emergency room after being found in a ditch where he had lain overnight after being hit by a car. He complains of severe pain in the left arm and examination reveals a broken humerus. Neurological examination reveals that the patient can extend the elbow but displays inability to supinate the elbow when it is extended. The patient also has wrist drop and very weak hand grasp. The neurological lesion is likely localized at which of the following locations?

  A. posterior cord of the brachial plexus

  B. posterior divisions of the brachial plexus

  C. radial nerve at the distal third of the humerus

  D. radial nerve at the midforearm

  E. radial nerve at the wrist

  Correct Answer: C

  Section: Anatomy Wrist drop and weak hand grasp indicate that the patient has a lesion of the radial nerve, most likely at the distal third of the humerus, sparing the innervation of the tr韈eps brachii but affecting the supinator. Because the patient can extend the elbow, the integrity of the posterior cord (choice A) and posterior divisions (choice B) of the brachial plexus is preserved. Lesion of the radial nerve at the midforearm (choice D) and wrist (choice E) would spare the innervation of the supinator.

• Question 793:

  Which of the following is characterized by an absence of lymphoid follicles and germinal centers?

  A. axillary lymph node

  B. Peyer's patches

  C. pharyngeal tonsil

  D. spleen

  E. thymus

  Correct Answer: E

  Section: Anatomy The thymus provides for development of new T lymphocytes in an environment shielded from foreign antigens. Like bone marrow, the thymus is a primary lymphoid organ, and not a site of reactions to foreign antigens. Lymphoid follicles are sites of B lymphocyte proliferation in response to antigen stimulation. These occur in the lymph nodes (choice A), Peyer's patches (choice B), tonsils (choice C), and white pulp of the spleen (choice D).

• Question 794:

  A gang member is rushed to the emergency room suffering from multiple stab wounds made by an ice pick. Athird-year medical student rotating through emergency medicine is puzzled by the sight of a milky white substance exuding from a stab wound just superior to the right sternoclavicular joint. Which of the following structures is possibly injured at this location?

  A. the common carotid artery

  B. the cupola of the right lung

  C. the internal jugular vein

  D. the right lymphatic trunk

  E. the thoracic duct

  Correct Answer: D

  Section: Anatomy The right lymphatic trunk is ruptured and chyle is exuding from the wound. Blood would flow from a wound involving the common carotid artery (choice A) or internal jugular vein (choice C). Astabbing lesion of the cupola of the right lung (choice B) would result in pneumothorax. The thoracic duct (choice E) does carry chyle. However, the thoracic duct drains into the bifurcation of the internal jugular and subclavian veins on the left side, above the left sternoclavicular joint.

• Question 795:

  A 72-year-old male patient has suffered a cerebral infarct affecting the left cerebral hemisphere. On examination of the patient, it is noticed that his verbal output is fluent and paraphasic. His comprehension of speech is normal but repetition is severely impaired. Naming is also impaired, although when given a list, the patient is able to select the correct name. Special consultation with a neurologist results in a diagnosis of conduction aphasia. Which of the following brain structures is affected?

  A. arcuate fasciculus

  B. Broca's area

  C. nucleus ambiguous

  D. red nucleus

  E. Wernicke's area

  Correct Answer: A

  Section: Anatomy The arcuate fasciculus connects Wernicke's area to Broca's area, integrating comprehension of speech with motor speech. A lesion in this connecting bundle results in this unusual condition of conduction aphasia, characterized by impairment of repetition and naming. Broca's area (choice B) is the motor speech area and lesion in this area will result in motor speech impairment. However, this patient's verbal output is fluent. Alesion in the nucleus ambiguus (choice C) will result in dysarthria and also loss of a fluent verbal output. The red nucleus (choice D) is a mesencephalic motor nucleus, which does not participate in the central pathways for speech. Wernicke's area (choice E) is the speech comprehension area, which is intact in this patient because he comprehends spoken language.

• Question 796:

  Arrow 2 in following figure points to which of the following structures?

  A. glomerulus

  B. juxtaglomerular cells

  C. lacis cells

  D. macula densa

  E. vascular pole of the renal corpuscle

  Correct Answer: D

  Section: Anatomy The macula densa indicated by arrow 2 is a modified segment of the distal tubule at the site of its passage adjacent to the vascular pole of the renal corpuscle. The name arises from the close spacing of nuclei of the epithelial cells forming this part of the distal tubule. These cells are thought tosense the chloride content in the passing filtrate and generate signals that regulate the caliber of the afferent arteriole. The glomerulus (choice A, arrow 1) is a network of capillaries derived from the afferent arteriole and suspended within Bowman's capsule by its vascular pole. The juxtaglomerular cells (choice B) form a cuff around the afferent arteriole prior to its entrance into the glomerulus and they secrete renin. Lacis cells (choice C, arrow 4) are extraglomerular mesangial cells with an uncertain function. The vascular pole (choice E, arrow 3) feeds blood to the glomerulus and carries blood away from it. As part of the juxtaglomerular apparatus, it is closely apposed to the macula densa.

• Question 797:

  In the central auditory pathways, second-order neurons are located in which of the following?

  A. cochlear (spiral) ganglion

  B. cochlear nuclei

  C. inferior colliculi

  D. nuclei of lateral lemniscus

  E. superior olivary nuclei

  Correct Answer: B

  Section: Anatomy Second-order neurons in the central auditory pathways are located in the dorsal and ventral cochlear nuclei. They receive afferents from the first-order neurons located in the cochlear (spiral) ganglion (choice A). Secondorder fibers from the cochlear nuclei project in turn to the inferior colliculi (choice C), nuclei of lateral lemniscus (choice D), and superior olivary nuclei (choice E).

• Question 798:

  A 15-year-old high school football player is brought to the emergency room. On examination, his right lower limb is deformed and swollen around the knee. At full extension, there is valgus instability, suggestive of knee dislocation. The patient is in great pain and there is a concern for concomitant vascular and nervous injuries. Palpation of the dorsalis pedis artery reveals a normal pulse. However, neurological examination reveals impaired dorsiflexion of the foot with decreased sensation in the space between digits 1 and 2. Which of the following nerves is affected?

  A. deep peroneal (fibular) nerve

  B. femoral nerve

  C. saphenous nerve

  D. superficial peroneal (fibular) nerve

  E. tibial nerve

  Correct Answer: A

  Section: Anatomy The deep peroneal (fibular) nerve innervates the muscles responsible for dorsiflexion in the anterior compartment of the leg. It also provides for the cutaneous innervation of the space between digits 1 and

  2. The femoral nerve (choice B) innervates muscles in the anterior compartment of the thigh and the skin of the medial aspect of the leg by a continuing branch, the saphenous nerve (choice C). The superficial peroneal (fibular) nerve (choice D) innervates the lateral compartment of the leg and muscles responsible for foot eversion. The tibial nerve (choice E) innervates the posterior compartment of the leg and the muscles responsible for plantar flexion.

• Question 799:

  A patient has been admitted for hematemesis (vomiting of blood). Endoscopic examination reveals bleeding esophageal varices resulting from portal obstruction. These varices represent anastomoses between branches of which of the following?

  A. inferior vena cava with a patent ductus venosus

  B. left gastric, azygos, and hemiazygos veins

  C. right gastric vein and the inferior vena cava

  D. superior, middle, and inferior rectal veins

  E. veins running on the ligamentum teres and the epigastric veins

  Correct Answer: B

  Section: Anatomy Obstruction of the portal vein results in an increase in the collateral circulation between veins that normally drain to the portal vein and those that drain to the systemic veins. Choices A, B, D, and E all represent possible collateral venous circulation in case of portal obstruction. Choice Ais rare because the ductus venosus closes after birth. Choice B is correct because varicose veins in this region give rise to esophageal varices. Choice D results in varicose veins in the rectal region. Choice C is incorrect because there is no connection between the right gastric vein and the inferior vena cava. In choice E, enlargement of the epigastric veins results in varicose veins radiating from the umbilicus, the caput medusae.

• Question 800:

  A 45-year-old female patient presents to the emergency room with a headache and complains of abnormal sensations on the left side of her body. She claims that the sensory changes came on rapidly in the last few hours. Her laboratory reports come back normal, a spinal tap reveals normal cerebrospinal fluid (CSF), and her mental ability seems good. You perform a neurological examination and find she has greatly reduced sensation on the entire left side of her body, including her face. The sensory loss includes all modalities. The motor examination is normal, as is her visual examination and hearing examination. Based on the neurological findings, which of the following thalamic nuclei would be involved?

  A. lateral and medial geniculate nuclei

  B. lateral dorsal and lateral posterior nuclei

  C. pulvinar

  D. ventral anterior and ventral lateral nuclei

  E. ventral posterior lateral and ventral posterior medial nuclei

  Correct Answer: E

  Section: Anatomy The ventral posterior lateral (VPL) and ventral posterior medial (VPM) nuclei are sensory relay thalamic nuclei. The lateral and medial geniculate nuclei (choice A) are respectively concerned with visual and auditory sensory information. The lateral dorsal (LD) and lateral posterior (LP) nuclei (choice B), and the pulvinar (choice C) are association or multimodal relay nuclei. The ventral anterior (VA) and ventral lateral (VL) are motor relay nuclei.