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USMLE STEP1 Exam Questions
Step1 (Page 28 )

Updated On: 31-Mar-2026
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PDF with 845 Questions

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.



Lymph nodes are populated by lymphocytes that exit the vascular compartment to gain access to the parenchyma of the node by passing through the walls of which of the following?

  1. afferent lymphatic vessels
  2. arterioles
  3. efferent lymphatic vessels
  4. high endothelial postcapillary venules
  5. medullary sinuses

Answer(s): D

Explanation:

High endothelial venules (HEV), located primarily in the deep cortex, are specialized to recruit circulating lymphocytes from the blood. Lymphocytes in the circulating blood adhere to the lining endothelial cells of HEV by way of an integrin-based recognition. Lymphocytes then gain access to the lymph node tissue by actively migrating (a process called diapedesis) between or through endothelial cells. Afferent lymphatic vessels (choice A) conduct lymph, not blood, into the lymph node. The source of the lymph is either upstream lymph nodes or tissue fluid from the region supplied by the node. This component of the system serves as a filter and as a mechanism for antigen-presenting cells to enter the node. Arterioles (choice B) are a component of the circulation of the lymph node, but they are not permeable to cell traffic. Efferent lymphatic vessels (choice C) conduct lymph and cells from the lymph node to either the blood circulation or downstream lymph nodes. Lymph in efferent lymphatic vessels conveys immunoglobulins and recirculating lymphocytes to the bloodstream. Medullary sinuses (choice E) are part of a system of passages that filter lymph and direct it from the afferent lymphatic vessels to the efferent lymphatic vessels. Medullary sinuses occupy spaces between medullary cords, which are occupied by large numbers of plasma cells, the cells that secrete immunoglobulins.



A 32-year-old female professional gardener complains of increasing muscle weakness and fatigue during the day, requiring her to take frequent rests. She also reports that she cannot enjoy her meals any more because her muscles of mastication quickly weaken and she has to stop chewing. When she watches television at night for a long period of time, her vision becomes blurry and she sees double. Her neurologist makes a preliminary diagnosis of myasthenia gravis. Which of the following is the cause of myasthenia gravis?

  1. Acetylcholine synthesis in motor neurons is impaired.
  2. Acetylcholinesterase synthesis is inhibited.
  3. Autoantibodies destroy cholinergic receptors at the postsynaptic membrane preventing binding of acetylcholine.
  4. Neurotransmitter release is impaired at the presynaptic membrane of the neuromuscular junction.
  5. Signal transduction within the muscle is impaired.

Answer(s): C

Explanation:

Myasthenia gravis is an autoimmune disorder where autoantibodies target the postsynaptic cholinergic receptors and destroy them. Acetylcholine released from motor neurons is thus unable to bind and the muscle contraction weakens due to decreased neurotransmitter communication. Acetylcholine synthesis in motor neurons (choice A) remains normal. Acetylcholinesterase (choice B) is the enzyme which degrades acetylcholine and its synthesis is not affected in myasthenia gravis. The cholinergic neurotransmitter release mechanism (choice D) at the presynaptic membrane, as well as the signal transduction mechanism (choice E) within the muscle, remain normal.



In Fig. following figure which labeled bracket spans a sarcomere?

  1. A
  2. B
  3. C
  4. D
  5. E

Answer(s): A

Explanation:

An individual sarcomere, the unit of contraction in striated muscle, spans the interval between successive Z lines. Each sarcomere encompasses an A-band (choice B) and half of each of two I bands (choice D).
Each myofibril (choice C) of a striated muscle fiber is composed of a tandem series of sarcomeres.
Coupling of excitation and contraction is a critical function of the triad (choice E), which is composed of a T tubule interposed between two cisternae of the sarcoplasmic reticulum.



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