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USMLE STEP1 Exam (page: 1)
USMLE Step1
Updated on: 12-Feb-2026

Viewing Page 1 of 171
STEP1 PDF 845 Questions

A 2-year-old male patient develops progressive generalized weakness and muscle atrophy. The impairment first began with the muscles of the hips, and then progressed to the pelvic area, thigh, and shoulder muscles. The patient is diagnosed with Duchenne's muscular dystrophy, a congenital disorder where the protein dystrophin is deficient. Which of the following describes the role of dystrophin in muscle tissue?

  1. anchors actin to the sarcolemma
  2. endows the myosin filaments with elastic recoil properties
  3. extends from Z disk to Zdisk, forming a supportive network
  4. inhibits the binding of myosin to actin
  5. protects desmin filaments from stressinduced damage

Answer(s): A

Explanation:

Dystrophin anchors actin to the sarcolemma, reinforcing and stabilizing the latter during muscle contraction. Titin is a large protein which associates with myosin filaments and endows them with elastic recoil properties (choice B). Desmin filaments form a supportive network extending from Z disk to Zdisk (choice C). Troponin I inhibits the binding of myosin to actin (choice D). AlphaB-crystallin protects desmin filaments from stress-induced damage (choice E).



The third week of development is characterized by the appearance of the branchial apparatus, the embryonic primordium from which head and neck structures will be derived. The apparatus consists of five branchial arches, numbered 1, 2, 3, 4 and 6. Second arch anomalies represent 95% of all branchial anomalies and are classified into four types with types IIII being the most common. The anomalies manifest as cysts or fistulae in the lateral neck, located anterior and deep to the sternocleidomastoid muscle. Which of the following structures develop from the second branchial arch?

  1. anterior digastric muscle
  2. posterior cricoarytenoideus muscle
  3. posterior digastric muscle
  4. stylopharyngeus muscle
  5. superior constrictor muscle

Answer(s): C

Explanation:

The posterior digastric muscle is derived from the second branchial arch and thus is innervated by its nerve, the facial (Seventh cranial) nerve. The anterior digastric muscle (choice A) is derived from the first branchial arch and is innervated by its nerve, the mandibular division of the trigeminal (fifth cranial) nerve.
The posterior cricoarytenoideus muscle (choice B) and the superior constrictor muscle (choice E), derived from a combination of the fourth and sixth branchial arches, are innervated by the vagus (tenth cranial) nerve. The stylopharyngeus (choice D) is derived from the third branchial arch and is the only voluntary muscle innervated by the glossopharyngeal (ninth cranial) nerve.



A 10-year boy is examined because his parents noticed that "his eyes never seem to look in the right direction." On examination, the left eye of the child is unable to move laterally (abduction) and when asked to look toward the nose (adduction), the eyeball retracts into the socket and the eye opening narrows. Sometimes, the eye also moves superiorly. The child is diagnosed with Duane syndrome, a congenital ocular motility disorder characterized by limited abduction of the affected eye. This is due to absence of the abducens (fifth cranial) nerve with aberrant innervation by the oculomotor (third cranial) nerve. Which of the following muscles is normally innervated by the abducens nerve?

  1. inferior oblique
  2. lateral rectus
  3. medial rectus
  4. superior oblique
  5. superior rectus

Answer(s): B

Explanation:

The abducens (sixth cranial) nerve normally innervates the lateral rectus muscle. The inferior oblique (choice A), medial rectus (choice C), and superior rectus (choice E) are all innervated by the oculomotor (third cranial) nerve. The superior oblique (choice D) is innervated by the trochlear (fourth cranial) nerve. In Duane syndrome, the absence of lateral rectus innervations by the abducens (sixth cranial) nerve is replaced by an aberrant innervation of the oculomotor (third cranial) nerve. Thus when the patient is asked to adduct the affected eye, there is cocontraction of the lateral and medial recti with globe retraction.



Retinitis pigmentosa is a hereditary disorder, which affects the photoreceptors (the rods and the cones) in the retina. These photoreceptors are located in which of the numbered layers in Figure below

  1. 1
  2. 2
  3. 3
  4. 4
  5. 5

Answer(s): E

Explanation:

The retina contains three layers of cells. They are, from top to bottom, the ganglion cells (choice A), the bipolar cells (choice C), and the photoreceptor cells (rods and cones, choice E). The internal plexiform layer (choice B) contains the synapses between the bipolar cells and the ganglion cells. The external plexiform layer (choice D) contains the synapses between the photoreceptors and the bipolar cells.
Remember that light enters from the top and traverses all the layers to reach the photoreceptors in the bottom layer.



The structure indicated by arrow 1 in Fig. 1-2 is which of the following vessels?

  1. brachiocephalic artery
  2. left brachiocephalic vein
  3. left common carotid artery
  4. right brachiocephalic vein
  5. superior vena cava

Answer(s): D

Explanation:

Remember that in viewing axial or transverse CT scans through the body, the right side of the patient is to your left and the left side to your right. In other words, the feet of the patient are toward you and the head away from you. The back of the patient is at the bottom of the image and the front of the patient toward the top. Directional terms are always in reference to the patient. The insert at the bottom right indicates the level of the section. Arrow 1 indicates the right brachiocephalic vein. The left brachiocephalic vein (choice B) is seen as the elongated structure immediately posterior to the manubrium of the sternum and to the left of the right brachiocephalic vein. Immediately posterior to the left brachiocephalic vein is the brachiocephalic artery (choice A, arrow 2). To the left of the latter are the left common carotid artery (choice C) and the left subclavian artery (arrow 3). The superior vena cava (choice E) is not seen at this level because the right and left brachiocephalic veins are still separate.



Viewing Page 1 of 171



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