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USMLE STEP1 Exam (page: 25)
USMLE Step1
Updated on: 16-Feb-2026

Viewing Page 25 of 171
STEP1 PDF 845 Questions

A newborn infant displays wheezing respiration, which is aggravated when she feeds, flexes her neck, and/ or cries. Radioimaging studies of her chest reveal a double aortic arch compressing her trachea and esophagus. This rare developmental defect results from persistence of the right dorsal aorta, which normally disappears. The arch of the aorta arises from which of the following structures?

  1. fifth pair of aortic arches
  2. fourth pair of aortic arches
  3. second pair of aortic arches
  4. sixth pair of aortic arches
  5. third pair of aortic arches

Answer(s): B

Explanation:

The arch of the aorta is formed from the left fourth aortic arch. Part of the right fourth aortic arch becomes the proximal portion of the right subclavian artery, whereas the rest of the fourth arch disappears. However, if it persists, a right aortic arch is formed passing posterior to the trachea and esophagus. With the formation of the normally occurring left aortic arch, which runs anterior to the trachea and esophagus, a double aortic arch is created. This defect clamps the trachea and esophagus resulting in the respiratory symptoms. The second pair of aortic arches (choice C) partially form the stapedial arteries in the middle ear of the embryo. The third pair of aortic arches (choice E) form the common carotid arteries and contribute to the internal carotid arteries. The fifth pair of aortic arches (choice A) either does not develop or form primitive vessels, which disappear eventually. The sixth pair of aortic arches (choice D) contribute to the formation of the pulmonary arteries and ductus arteriosus.



After recovery from the surgical removal of the thyroid gland, the quality of the voice in a patient changed to a monotone. It was discovered that the cricothyroid muscle in this patient was paralyzed. The cricothyroid muscle is innervated by which of the following?

  1. accessory nerve
  2. ansa cervicalis
  3. external laryngeal nerve
  4. hypoglossal nerve
  5. internal laryngeal nerve

Answer(s): C

Explanation:

The external laryngeal nerve, a branch of the superior laryngeal nerve from the vagus, innervates the cricothyroid muscle. Normally, the cricothyroid muscle varies the length and tension of the vocal cord, and in its absence, the voice acquires a monotonous quality. The accessory nerve (XI, choice A) only innervates the sternocleidomastoid and the trapezius muscles in the neck. The ansa cervicalis (choice B) supplies motor branches to the infrahyoid muscles. The hypoglossal nerve (XII, choice D) innervates intrinsic and extrinsic muscle fibers of the tongue. The internal laryngeal nerve (choice E) provides sensory innervation to the interior of the larynx.



Secretion of pulmonary surfactant is a function of which of the following?

  1. alveolar dust cells
  2. endothelial cells of capillaries in the alveolar septum
  3. small granule cells
  4. type I pneumocytes (squamous alveolar cells)
  5. type II pneumocytes (greater alveolar cells)

Answer(s): E

Explanation:

All the listed cell types are components of the respiratory system. Type II pneumocytes are the source of pulmonary surfactant. Alveolar dust cells (choice A) are macrophages. Endothelial cells (choice B) and type I pneumocytes (choice D) are components of the blood-air barrier. Small granule cells (choice C), which are members of the diffuse neuroendocrine system, function in paracrine and endocrine signaling.



Climbing fibers provide excitation to the deep cerebellar nuclei and the Purkinje neurons. The climbing fibers originate from which of the following structures?

  1. inferior olivary nucleus
  2. pontine nuclei
  3. spinal cord
  4. superior olivary nucleus
  5. vestibular nuclei

Answer(s): A

Explanation:

The inferior olivary nucleus in the medulla oblongata is the sole source of climbing fibers to the cerebellum.
All other sources of fibers to the cerebellum such as the pontine nuclei (choice B), spinal cord (choice C), or vestibular nuclei (choice E) provide mossy fibers to the cerebellum. The superior olivary nucleus (choice D) receives auditory fibers from the cochlear nuclei and projects back to the cochlea by way of the olivocochlear bundle. It modifies the sensory input from the organ of Corti in the cochlea and is not involved with cerebellar circuitry.



A newborn infant suffers from epidermolysis bullosa simplex, characterized by skin blistering soon after birth. The blisters appear at sites of pressure or rubbing such as the fingers. You explain to the distressed parents that this condition is a disease of intermediate filaments and results from mutant genes for keratins K5 and K14. Which cells of the skin normally produce keratins K5 and K14?

  1. adipose cells in the hypodermis
  2. basal cells of the epidermis
  3. fibrocytes of the dermis
  4. melanocytes of the epidermis
  5. upper differentiating cells of the epidermis

Answer(s): B

Explanation:

Basal cells of the epidermis produces keratins K5 and K14. Adipose cells in the hypodermis (choice A), fibrocytes of the dermis (choice C), and melanocytes of the epidermis (choice D) do not synthesize keratins. Upper differentiating cells of the epidermis (choice E) produce keratins K1 and K10. A keratin K9 is also found in the palmar or plantar regions of the epidermis.



Viewing Page 25 of 171



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