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USMLE STEP1 Exam Questions
Step1 (Page 19 )

Updated On: 31-Mar-2026
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A 52-year-old male patient with a history of alcoholism is brought to the emergency room because of a sudden onset of right upper quadrant pain, nausea, blood in the vomitus, and fever. Ultrasound diagnosis reveals a portal vein obstruction caused by liver cirrhosis and accompanied by esophageal varices. Proper portal circulation must be reestablished to alleviate the esophargeal varices, and the attending surgeon decides on an end-to-side portocaval shunt. Which of the following describes the chosen procedure?

  1. anastomosis by communication between the portal vein and the inferior vena cava
  2. anastomosis by prosthetic vascular graft between the inferior vena cava and the superior mesenteric vein
  3. anastomosis by suturing the inferior end of the portal vein to the inferior vena cava
  4. anastomosis of the splenic vein to the left renal vein
  5. placement of an expandable stent between a hepatic vein and the portal vein

Answer(s): C

Explanation:

Suturing the inferior end of the portal vein to the inferior vena cava is termed end-to-side portocaval shunt, whereas establishing a communication between the portal vein and the inferior vena cava (choice A) is a side-to-side portocaval shunt. Placing a prosthetic vascular graft between the inferior vena cava and the superior mesenteric vein (choice B) is termed a mesocaval shunt. Anastomosis of the splenic vein to the left renal vein (choice D) is a splenorenal or Warren shunt. Placement of an expandable stent between the hepatic vein and portal vein (choice E) is called a transjugular intrahepatic portosystemic shunt or TIPS.



During surgical resection of the gastrointestinal tract for treatment of long-standing irritable bowel syndrome, a surgery resident is asked to select a surgical site coinciding with the termination of the vagal innervation. Which of the following does she select?

  1. duodenojejunal junction
  2. ileocolic junction
  3. junction of the second part and third part of the duodenum
  4. left colic (splenic) flexure
  5. right colic (hepatic) flexure

Answer(s): D

Explanation:

The left colic (splenic) flexure represents the junction of the midgut to the hindgut during embryonic development. The vagus (tenth cranial) nerve provides the parasympathetic innervation of the digestive tract up to this flexure, whereas the pelvic parasympathetic fibers from S2-S4 innervate the rest of the tract.
The duodenaljejunal junction (choice A), ileocolic junction (choice B), and rightcolic (hepatic) flexure (choice E) are anatomical landmarks along the digestive tract without change in parasympathetic innervation. The junction of the second part and third part of the duodenum (choice C) represents the embryonic junction between the foregut and the midgut, but both receive vagal innervation.



The structure indicated by arrow 1 in following figure is innervated by which of the following?

  1. anterior ethmoidal nerve
  2. greater palatine nerve
  3. lesser palatine nerve
  4. middle superior alveolar nerve
  5. nasopalatine nerve

Answer(s): D

Explanation:

Arrow 1 points to the maxillary sinus, which is innervated by the posterior, middle, and anterior superior alveolar nerves. The latter are branches from the infraorbital nerve arising from the maxillary division (V2) of the trigeminal (fifth cranial) nerve. The anterior ethmoidal nerve (choice A) is a branch of the nasociliary nerve from the ophthalmic division (V1) of the trigeminal nerve. The greater palatine nerve (choice B) and lesser palatine nerve (choice C) are branches of the maxillary division of the trigeminal nerve supplying the hard and soft palate, respectively. The nasopalatine nerve (choice E) is derived from the maxillary division of the trigeminal nerve and supplies the nasal septum.



Which of the following thalamic nuclei subserves a motor function?

  1. dorsomedial nucleus
  2. lateral geniculate nucleus
  3. medical geniculate nucleus
  4. ventral lateral nucleus
  5. ventral posterior medial nucleus

Answer(s): D

Explanation:

The ventral lateral nucleus of the thalamus receives projections from the deep cerebellar nuclei and, in turn, sends projections to the motor cortex. It is thus functionally defined as a thalamic motor nucleus. The dorsomedial nucleus (choice A) is considered a limbic nucleus of the thalamus as it receives projections from the amygdala and olfactory cortex, and projects to the prefrontal cortex and hypothalamus. The lateral (choice B) and medial geniculate (choice C) nuclei and the ventral posterior medial nucleus (choice D) are considered sensory nuclei as they receive information mediating vision, audition, and somatosensory signals from the face, respectively. In turn, these three brain areas project to the somatosensory cortex.



A neurology resident is testing the jaw-jerk reflex in a patient by tapping gently on the right masseter muscle and observing elevation of the mandible. What is the location of the neuronal cell bodies of the proprioceptive fibers mediating the jaw-jerk reflex?

  1. mesencephalic trigeminal nucleus
  2. motor trigeminal nucleus
  3. principal (main) trigeminal nucleus
  4. spinal trigeminal nucleus
  5. trigeminal (gasserian) ganglion

Answer(s): A

Explanation:

The jaw-jerk reflex is a monosynaptic (stretch) reflex for the masseter muscle. Proprioceptive fibers from the muscle travel by way of the trigeminal nerve back to their cell bodies in he mesencephalic trigeminal nucleus. Projections from this nucleus synapse on motor neurons of the motor trigeminal nucleus (choice B) which elicits contraction of the masseter muscle. The principal (main) trigeminal nucleus (choice C) receives light touch sensory information from the face, whereas the spinal trigeminal nucleus (choice D) receives pain and temperature sensations. The trigeminal (gasserian) ganglion (choice E) contains the cell bodies of all sensory neurons projecting to the principal (main) and spinal trigeminal nuclei.



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