USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 701:

  A very thin 15-year-old female presents with headache, polyuria, and grand mal seizures. She answers the question, if she has had to vomit recently, with "frequently" but that these symptoms "are under control now." Her body mass index is 14.1. Serum glucose, calcium, and potassium are normal. Serum sodium, chloride, and osmolarity are low. As a working diagnosis, which of the following is most likely?

  A. Addison disease

  B. central diabetes insipidus

  C. diabetes mellitus

  D. nephrogenic diabetes insipidus

  E. water intoxication

  Correct Answer: E

  Section: Physiology Excessive vomiting can lead to large losses in fluid and electrolytes. Dehydration triggers the sense of thirst. Her age, her extremely low body mass index (normal 18.524.9), and her frequent vomiting all point toward anorexia nervosa. Anorexic people might drink large amounts of water to reach target weight. By drinking a lot of water without adequately replacing electrolyte imbalances, water intoxication can result. Diluted serum sodium levels can lead to headaches and seizures. Although vomiting, weight loss, and fainting due to low blood pressure could point toward Addison disease, the typical symptoms of hypocortisolism, which include hyperkalemia, hyperosmolality, and hyperpigmentation, are not mentioned and hence make choice A not the best one. In patients with diabetes insipidus, whether caused by a lack of ADH (choice B) or by failure of the kidney to adequately respond to ADH (choice D), serum osmolality is expected to be high. Normal glucose values make uncontrolled diabetes mellitus not the best working diagnosis (choice C).

• Question 702:

  End-stage chronic renal disease is associated with a glomerular filtration rate (GFR) of less than 15 mL/ min and an increase in which of the following blood values?

  A. calcium

  B. erythropoietin (EPO)

  C. hematocrit

  D. phosphate

  E. 1,25-(OH vitamin

  Correct Answer: D

  Section: Physiology Chronic renal failure reduces phosphate excretion and thus phosphate accumulates in the blood. The renal peritubular cells are the major source of circulating EPO, which stimulates red blood cell production. Hence renal failure will decrease EPO levels (choice C) and will decrease hematocrit (choice E) as well. The kidney is responsible for production of vitamin D3, the active form of vitamin D, and renal failure also results in decreased active vitamin D (choice E). Secondary to this, the circulating levels of calcium will decrease (choice A).

  

• Question 703:

  A 34-year-old male is brought into the emergency room, having been found unconscious in his apartment. Apparently, he has been in this state for 2 days. The laboratory values for the patient's blood and urine are consistent with elevated circulating antidiuretic hormone (ADH). Which one of the following will directly stimulate ADH in this patient?

  A. angiotensinogen

  B. extracellular fluid osmolality increase

  C. temperature decrease

  D. thyroid hormone

  E. volume increase

  Correct Answer: B

  Section: Physiology ADH acts to increase renal water conservation secondary to dehydration. Choice B, which can result from dehydration, is the logical trigger for ADH secretion. Angiotensinogen (choice A) is inactive and will not directly stimulate ADH, but its active metabolite angiotensin II or III will increase ADH secretion. Choices C, D, and E will either inhibit, or have minimal effects on ADH secretion.

• Question 704:

  A healthy 17-year-old male tells his doctor during a normal check-up visit that "his muscles gave out" when he recently attempted to lift a heavy load. On questioning, he answers that he did not experience any other neuromuscular problems after this experience. What is the most likely mechanism responsible for the abrupt cessation of skeletal muscle contraction?

  A. activation of alpha motor neurons

  B. activation of gamma motor neurons

  C. activation of Golgi tendon organs

  D. activation of muscle spindles

  E. skeletal muscle ischemia

  Correct Answer: C

  Section: Physiology When there is danger of damage to the tendons or bones to which the muscles are attached, Golgi tendon organs are activated and elicit rapid responses through neural connections located within the spinal cord that result in muscle relaxation. Golgi tendon organs are muscle proprioceptors that are found close to the junction between tendon and muscle fibers. They are in series with the muscle fibers and respond to the stretch of tendons which accompanies muscle tension. The threshold for activating them in this manner is high and it is therefore believed that they play an important role in the reflex response of a muscle after excessive force ("muscles give out"). In the spinal reflex pathways, the Ib afferent axons from the Golgi tendon organ synapse on inhibitory spinal cord interneurons, which then inhibit, not activate, alpha motor neuron activity serving the same muscles (choice A). Gamma motor neurons (choice B) innervate muscle spindles. These neurons control the sensitivity of the spindle by maintaining the proportions between spindle and muscle length and by dampening the function of the spindle, regardless of change in muscle length. This is important for normal movements. Muscle spindles (choice D) continuously signal information about the length of a muscle and the rate of change in length. They are important for conscious appreciation of the body's position, for planning and execution of controlled movements. Low oxygenation of the muscle or ischemia (choice E) is not the primary trigger for spinal reflexes, and will occur gradually, not suddenly as described in the question.

• Question 705:

  The hands and nails of a 45-year-old woman show the discolorations indicated in below figure. The woman states that she had pulmonary tuberculosis about 10 years ago. The clinician prescribes cortisol, but counsels her that as a side effect it might lead to thin skin and thinning of her limbs because of an increase in which of the following?

  

  A. adrenocorticotropic hormone (ACTH) secretion

  B. corticotrophin-releasing hormone (CRH) secretion

  C. insulin sensitivity in muscle

  D. protein degradation

  E. wound healing

  Correct Answer: D

  Section: Physiology Tuberculosis is known to increase the susceptibility of developing chronic adrenal insufficiency, also known as hypocortisolism, or Addison disease. It leads to hyperpigmentation as seen in the palmar skin creases and nails of the woman. Administration of cortisol replaces the inadequate release of glandular cortisol. Cortisol decreases protein synthesis and increases protein degradation. The newly available amino acids are then used for gluconeogenesis, which lead to the name "glucocorticoid" for cortisol. Increasing serum cortisol levels will decrease, not increase, secretion of ACTH (choice A) and CRH (choice B) by feedback inhibition. Cortisol decreases, not increases, the sensitivity of cells to insulin (choice C) by decreasing the translocation of glucose transporters to the cell membrane. Because cortisol decreases utilization of glucose by muscle and adipose tissue, blood glucose rises. This provides a stimulus to release insulin. Thus, prolonged exposure to high levels of cortisol can lead to diabetes mellitus due to exhaustion of pancreatic beta cells. Although cortisol acts as an anti- inflammatory agent, it has a negative, not a positive, effect on wound healing (choice E). Cortisol suppresses the activity of leukocytes which are important for debriding the wound. Cortisol increases catabolism, which leads to the breakdown of nutrients that are needed for generating new tissue. Cortisol stimulates catecholamines leading to vasoconstriction and consequent reduction of oxygen, nutrient, and white blood cell delivery. Cortisol may also inhibit collagen production.

• Question 706:

  Askydiver landing forcefully on his right lower limb suffered a central fracture of the acetabulum with dislocation of the femoral head into the pelvis. The acetabulum is formed by the joining of the ilium, ischium, and pubis. These three bones are completely fused by which of the following periods?

  A. birth

  B. 6 years

  C. puberty

  D. 16 years

  E. 23 years

  Correct Answer: E

  Section: Anatomy Fusion of the ilium, ischium, and pubis at the acetabulum is usually complete by age 23. From birth to the early 20s (choices AD), the three bones are held together by a Y-shaped cartilage.

• Question 707:

  In the coronal section of the head shown in following figure arrow 2 points to a structure that belongs to which of the following?

  

  A. ethmoid bone

  B. inferior nasal concha

  C. maxillary bone

  D. nasal septum

  E. vomer bone

  Correct Answer: A

  Section: Anatomy Arrow 2 points to the middle nasal concha, which is part of the ethmoid bone. Arrow 3 points to the ethmoid sinus, which also belongs to the ethmoid bone. The inferior nasal concha (arrow 1) is a separate bone (choice B). The maxillary bone (choice C) is located more laterally and does not contribute to the nasal conchae. The maxillary sinus, a part of the maxillary bone, is indicated by arrow

  4. The vomer bone (choice E) forms part of the nasal septum (choice D), which can be seen here as the thin line between the nasal conchae. The nasal conchae have no attachment to the septum.

• Question 708:

  Apatient with a developing astroglioma in the primary auditory cortex (Brodmann's area 41) complains of hearing unusual noises. What is the main source of subcortical input to the primary auditory cortex?

  A. inferior colliculus

  B. lateral geniculate nucleus

  C. medial geniculate nucleus

  D. superior colliculus

  E. superior olivary nucleus

  Correct Answer: C

  Section: Anatomy The main subcortical afferents to the primary auditory cortex (Brodmann's area 41) arise from the medial geniculate nucleus of the thalamus. The inferior colliculus (choice A) is a midbrain auditory relay nucleus, which sends projections to the medial geniculate nucleus. The lateral geniculate nucleus (choice B) is a visual relay nucleus in the thalamus and it projects to the primary visual cortex (Brodmann's area 17). The superior colliculus (choice D) is a midbrain visual relay nucleus, which provides fibers to the tectospinal tract. The superior olivary nucleus (choice E) is a medullary auditory relay nucleus, which sends projections to the cochlea.

• Question 709:

  A 65-year-old male patient develops neurological symptoms due to an embolus in the left common carotid artery. In the CT scan in following figure, the involved vessel is indicated by which of the following arrows?

  

  A. 1

  B. 2

  C. 3

  D. 4

  E. 5

  Correct Answer: D

  Section: Anatomy Arrow 4 points to the left common carotid artery. The three arteries arising from the arch of the aorta are lined anteriorly to posteriorly on a slanted line: arrow 3 (choice C) shows the brachiocephalic artery, arrow 4 (choice D) points to the left common carotid artery, and arrow 5 (choice E) indicates the left subclavian artery. Arrow 2 (choice B) points to the right brachiocephalic vein, which is joined here by the left brachiocephalic vein. Arrow 1 (choice A) points to the trachea, which is filled with air and thus is not radiopaque.

• Question 710:

  A 53-year-old male patient has developed an abnormal hypersensitivity in the face area. He complains that he feels pain when he drives with the window down and from the wind blowing on his face. The pain is exacerbated by direct touch to the left side of his lower jaw. The patient is diagnosed with trigeminal neuralgia (tic douloureux). Where do pain fibers from the left side of his mandible project to?

  A. mesencephalic trigeminal nucleus

  B. motor trigeminal nucleus

  C. principal sensory trigeminal nucleus

  D. spinal (descending) trigeminal nucleus

  E. trigeminal (gasserian) ganglion

  Correct Answer: D

  Section: Anatomy Fibers from pain and temperature receptors in the face area all project to the spinal (descending) trigeminal nucleus. The mesencephalic trigeminal nucleus (choice A) contains the perikarya of the proprioceptive fibers of the muscles of mastication. The motor trigeminal nucleus (choice B) receives innervations from the mesencephalic trigeminal nucleus. The principal sensory trigeminal nucleus (choice C) receives somatosensory fibers from the face, gums, teeth, and anterior two-thirds of the tongue. The trigeminal (gasserian) ganglion (choice E) contains the sensory perikarya of all three subdivisions of the trigeminal nerve.