CompTIA
Checkpoint
ISC
HP
Dell
EC-Council
EXIN
Fortinet
ITIL®
Juniper
PMI
Microsoft
VMware
Avaya
Google
Salesforce
Amazon
Palo Alto Networks
Certiport
ISC2
All Vendors
  2. Home
  3. Exams
  4. USMLE
  5. STEP1

Free STEP1 Exam Braindumps (page: 34)

Page 33 of 213
PDF with 845 Questions

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.



During development, which of the following structures act as a temporary set of kidneys in the fetus?

  1. mesonephroi
  2. metanephroi
  3. paramesonephric ducts
  4. pronephroi
  5. ureteric bud

Answer(s): A

Explanation:

The second set or mesonephroi appear late in the fourth week and are functional until the permanent kidneys or metanephroi are fully developed. During development, three sets of kidneys are formed in the embryo. The first set or pronephroi (choice D) are transitional, nonfunctional structures that appear around the fourth week of development. Next come the mesonephroi, then the permanent set of kidneys develops from the metanephroi. The paramesonephric ducts (choice C) are structures developing lateral to the gonads and mesonephric ducts. They play an essential role in the female reproductive system, but are not involved in the formation of the kidneys. The ureteric bud (choice E) is an outgrowth from the mesonephric duct that gives rise to the ureter, renal pelvis, calices, and collecting tubules.



In adults, lack of vitamin D gives rise to the disease osteomalacia characterized by progressive softening and bending of the bones. This is due to a defect in the mineralization of the osteoid. Under normal conditions, the osteoid is found along which of the following locations?

  1. the interface between osteocytes and bones
  2. the interface between osteoprogenitor cells and bone marrow
  3. the interface between the fibroblasts in the periosteum
  4. the interface between the osteoblast and bone
  5. the ruffled border of osteoclasts

Answer(s): D

Explanation:

Osteoid is the unmineralized organic matrix formed by osteoblasts and found at the interface between these cells and bone. Osteocytes (choice A) are surrounded by bone and no longer manufacture osteoid.
Osteoprogenitor cells (choice B) are similar to stem cells and do not manufacture bone material.
Fibroblasts (choice C) are cells of connective tissue forming the periosteum and not bone. Osteoclasts (choice E) are bone-resorbing cells.






Post your Comments and Discuss USMLE STEP1 exam with other Community members:

STEP1 Discussions & Posts