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USMLE STEP1 Exam (page: 26)
USMLE Step1
Updated on: 16-Feb-2026

Viewing Page 26 of 171
STEP1 PDF 845 Questions

Cell adhesion molecules enable intercellular contacts in epithelial as well as nonepithelial cells. Which of the following cell adhesion molecules are calcium-dependent molecules?

  1. disintegrins
  2. integrins
  3. intercellular adhesion molecules (I-CAM)
  4. neural cell adhesion molecules (N-CAM)
  5. selectins

Answer(s): E

Explanation:

Cell adhesion molecules are divided into two major classes on the basis of their dependence on calcium.
Calcium-dependent cell adhesion molecules are the cadherins and the selectins. Calcium- independent cell adhesion molecules include members of the immunoglobulin superfamily, the I-CAM (choice C), and the N- CAM, as well as the integrins (choice B). Disintegrins (choice A) are found in snake venom and disrupt the integrin-mediated cell binding to extracellular matrix proteins such as laminin or fibronectin.



After removal of cancerous lymph nodes from the lateral pelvic wall, a patient develops painful spasms of the adductor muscles and sensory deficits in the medial thigh region. The adductor muscles are innervated by which of the following nerves?

  1. femoral
  2. inferior gluteal
  3. obturator
  4. pudendal
  5. sciatic

Answer(s): C

Explanation:

The obturator nerve innervates the adductor muscles and the medial region of the thigh. The nerve originates from the lumbar plexus, runs on the lateral aspect of the pelvic wall, and exits through the obturator canal to reach the medial aspect of the thigh. Lying on the lateral pelvic wall, it may be injured by surgical mishap. The femoral nerve (choice A) innervates the anterior aspect of the thigh and the muscles contained within the sartorius and the quadriceps femoris. The inferior gluteal nerve (choice B) innervates the gluteus maximus muscle and is confined to the gluteal region. The pudendal nerve (choice D) is sensory to the genitalia, motor to the perineal muscles, the external urethral sphincter, and the external anal sphincter. The sciatic nerve (choice E) innervates the hamstring muscles in the posterior aspect of the thigh.



A renal calculus (kidney stone) passing from the renal pelvis into the ureter causes excessive distention and severe ureteric colic. During development in the embryo, the ureter arose from which of the following?

  1. mesonephric duct
  2. metanephric diverticulum
  3. metanephric mass of intermediate mesoderm
  4. paramesonephric duct
  5. pronephric duct

Answer(s): B

Explanation:

The metanephric diverticulum or ureteric bud gives rise to the ureter, renal pelvis, calices, and collecting tubules. The metanephric mass of intermediate mesoderm (choice C) gives rise to the nephrons in the kidney. The mesonephric and paramesonephric ducts (choices Aand D) play essential roles in the development of the male and female reproductive system, respectively. The pronephric duct (choice E) is derived from the transitory, nonfunctional first set of kidneys or pronephroi and does not contribute to the development of the ureter.



A patient suffering from Charcot-Marie-Tooth disease displays progressive degeneration of peripheral nerves, distal muscle weakness and atrophy, and defects in deep tendon reflexes. This condition is associated with an abnormal mutation in the gene encoding connexin-32. Connexins are normally found in which type of cell junctions?

  1. communicating (gap) junction
  2. hemidesmosome
  3. macula adherens (spot desmosome)
  4. occluding (tight) junction
  5. zonula adherens (belt desmosome)

Answer(s): A

Explanation:

Communicating (gap) junctions are formed by connexins, which associate together in groups of six to form connexons. The alignment of connexions between two cells allows for direct channels of communication between their cytoplasms, facilitating the transfer of molecules such as calcium or cyclic adenosine monophosphate (cAMP). Hemidesmosome (choice B), macula adherens (spot desmosome; choice C), and zonula adherens (belt desmosome; choice E) are classified as anchoring junctions. They are associated with intermediate filaments (hemidesmosome and macula adherens) or with actin microfilaments (zonula adherens), but not connexins. Occluding (tight) junctions (choice D) contain the proteins occludin and claudin but not connexin.



Calcitonin decreases blood calcium and boneresorbing activity in which of the following?

  1. osteoblast
  2. osteoclast
  3. osteocyte
  4. osteon
  5. osteoprogenitor cells

Answer(s): B

Explanation:

Calcitonin reduces the surface ruffling of osteoclasts and their activity. Osteoclasts are formed by fusion of blood monocyte derivatives and are components of the mononuclear phagocyte system. Under the influence of parathyroid hormone, osteoclasts enlarge their ruffled borders and increase their bone- resorbing activity. Osteoprogenitor cells (choice E), osteoblast (choice A), and osteocyte (choice C) are bone-forming cells. Osteon (choice D) is another name for the haversian system which is the haversian canal, its contents, surrounding lamellae, and osteocytes.



Viewing Page 26 of 171



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