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USMLE STEP1 Exam (page: 19)
USMLE Step1
Updated on: 15-Feb-2026

Viewing Page 19 of 171
STEP1 PDF 845 Questions

Which of the following statements concerning muscle spindles is correct?

  1. Activation of type Ia sensory fibers from a given spindle leads to inhibition of the muscle in which that spindle is located.
  2. Alpha motoneurons synapse directly with intrafusal muscle fibers.
  3. Each intrafusal fiber is innervated by two different gamma motoneurons.
  4. Only one type of intrafusal muscle fiber (cell) is present in most muscle spindles.
  5. Type Ia sensory fibers from a spindle form direct synaptic contact with alpha motoneurons in the spinal cord.

Answer(s): E

Explanation:

The type Ia sensory fibers from a spindle form direct excitatory synapses with alpha motoneurons.
Activation of type Ia sensory fibers (choice A) leads to excitation of the muscle in which that spindle is located. Alpha motoneurons (choice B) synapse with extrafusal muscle fibers, whereas gamma motoneurons synapse with intrafusal muscle fibers. Each intrafusal muscle fiber (choice C) is innervated by only one gamma motoneuron. Each muscle spindle contains a mixture of both nuclear bag and nuclear chain intrafusal fibers, not just one type as indicated in choice D.



Which of the following is characteristic of damage to the corticospinal (pyramidal) system?

  1. Babinski's sign
  2. flaccid paralysis and hypotonia
  3. immediate muscle degeneration and atrophy
  4. intention tremor
  5. loss of deep tendon reflexes

Answer(s): A

Explanation:

The Babinski sign--dorsiflexion of the great toe in response to stroking the plantar aspect of the foot-- is a characteristic sign of pyramidal tract involvement. Signs and symptoms of corticospinal tract injury that are nearly always apparent to some degree include spastic paralysis, hypertonia, loss of deep tendon reflexes, and hyperactive abdominal and cremasteric reflexes. Flaccid paralysis and hypotonia (choice B) are commonly seen following lower motoneuron injury, as is loss of deep tendon reflexes (choice E). Muscle degeneration and atrophy (choice C) are not characteristic symptoms of corticospinal tract damage. The presence of an intention tremor (choice D) is a sign of cerebellar damage, and is not seen with corticospinal tract lesions.



Cilia are anchored to the apical portion of certain types of epithelial cells and they are mobile. Which of the following form the motile cytoskeletal component of the cilia?

  1. intermediate filaments
  2. microfilament
  3. microtubule
  4. neurofilament
  5. tonofilaments

Answer(s): C

Explanation:

Acilium is formed by a concentric array of nine microtubule doublets surrounding a central pair of microtubules (9 + 2 organization). Intermediate filaments (choice A) are intermediate in diameter size (10 nm) between the microtubules (25 nm) and the microfilaments (7 nm). They do not confer motility to the cilia. They instead participate in the cytoskeletal integrity of nerve cells, neurofilaments (choice D) or hemidesmosomes, tonofilaments (choice E). Microfilaments (choice B) are found in microvilli and stereocilia.



A 79-year-old female patient was admitted to the hospital, presenting with fever, vomiting, dehydration, and distension of the abdomen. An X-ray reveals ileus and exploratory surgery reveals occlusion of vasa recta of the jejunum. Which of the following arteries supply branches to the involved vasa recta?

  1. ileocolic artery
  2. inferior pancreaticoduodenal artery
  3. right colic artery
  4. right gastroepiploic artery
  5. superior mesenteric artery

Answer(s): E

Explanation:

Ileus is obstruction of the intestine, in this case due to paralysis resulting from ischemia. The jejunum receives its vascular supply from jejunal branches arising from the superior mesenteric artery. The ileocolic artery (choice A) supplies the ileum, ileocolic junction, and the appendix. The inferior pancreaticoduodenal artery (choice B) provides vascular innervations to the pancreas and duodenum, and forms the anastomosis between the foregut and midgut. The right colic artery (choice C) supplies the ascending colon and the right gastroepiploic artery (choice D) the greater curvature of the stomach and greater omentum.



Which of the following is directly involved with the descending modulation of pain transmission?

  1. dopamine
  2. MLF
  3. nucleus raphe magnus
  4. rubrospinal fibers
  5. ventral lateral thalamic nucleus

Answer(s): C

Explanation:

The nucleus raphe magnus receives input from the periaqueductal gray and gives rise to descending serotonergic fibers of the raphe spinal projection. The latter fibers activate enkephalinergic spinal cord interneurons that presynaptically inhibit incoming pain fibers at their initial synapse in the spinal cord dorsal horn. The neurotransmitter dopamine (choice A) has not been shown to be involved in the descending systems that modulate pain transmission. The MLF (choice B) is an ascending fiber system in the brainstem that is primarily involved in the control of eye movements. The rubrospinal system (choice D) is a descending fiber tract involved with the control of limb musculature. The ventral lateral nucleus of the thalamus (choice E) is primarily involved with motor function and does not contribute to descending pathways that influence pain transmission.



Viewing Page 19 of 171



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