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USMLE STEP1 Exam Questions
Step1 (Page 15 )

Updated On: 31-Mar-2026
Page 15 of 171
PDF with 845 Questions

In this same patient, the pectineus muscle is also badly bruised. This muscle is innervated by which of the following?

  1. genitofemoral nerve
  2. obturator nerve
  3. peroneal division of the sciatic nerve
  4. saphenous nerve
  5. tibial division of the sciatic nerve

Answer(s): B

Explanation:

The pectineus muscle is part of the adductor muscles of the thigh, which are innervated as a group by the obturator nerve. The genitofemoral nerve (choice A) provides sensory innervation for a small area of the skin in the medial superior aspect of the thigh, and motor innervation to the cremaster muscle. The peroneal division of the sciatic nerve (choice C) innervates the skin and muscles in the peroneal and anterior comparments of the leg. The saphenous nerve (choice D) is purely a sensory nerve for the medial aspect of the leg and the foot. The tibial division of the sciatic nerve (choice E) provides sensory and motor innervations to the posterior compartment of the leg and the plantar foot.



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?

  1. omohyoid
  2. sternohyoid
  3. sternothyroid
  4. stylohyoid
  5. thyrohyoid

Answer(s): D

Explanation:

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.



A 35-year-old male patient suffering from pulmonary hypertension has been diagnosed with ostium secundum atrial septal defect. Abnormal development of which of the following structures is responsible for this developmental defect?

  1. aorticopulmonary septum
  2. endocardial cushion
  3. interventricular septum
  4. septum primum
  5. sinus venosus

Answer(s): D

Explanation:

Abnormal development of either the septum primum or septum secundum results in ostium secundum atrial septal defects in the area of the fossa ovalis. This common type of congenital heart defect is manifested by a patent foramen ovale between right and left atria. This is well-tolerated during childhood, but symptoms usually appear after 30 years of age. The aorticopulmonary septum (choice A) divides the truncus arteriosus of the developing heart and gives rise to the ascending aorta and pulmonary trunk. The endocardial cushions (choice B) give rise to the right and left atrioventricular canals. The interventricular septum (choice C) forms between the right and left ventricles. The sinus venosus (choice E) becomes incorporated into the atria, as well as giving rise to the openings of the pulmonary veins and the venae cavae.



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?

  1. type I
  2. type III
  3. type IV
  4. type V
  5. type VI

Answer(s): B

Explanation:

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.



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

  1. blood vessels
  2. differentiating blood cell precursors
  3. osteoblasts
  4. osteoclasts
  5. osteocytes

Answer(s): E

Explanation:

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.



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