USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 631:

  The eye examination of a patient with diabetes mellitus reveals no cataract or glaucoma. The patient's ophthalmoscopic picture of his left eye is shown in the below figure. Which of the following is most likely affected?

  

  A. color vision

  B. high acuity vision

  C. near point vision

  D. scotopic vision

  E. spatial vision

  Correct Answer: D

  Section: Physiology The picture, representing the back of the retina as seen through the ophthalmoscope, shows a normal optic disk, a normal macula lutea, and no obvious neovascularization. On the other hand, there are dot hemorrhages as well as one flame-shape hemorrhage site present in the peripheral retina. This indicates an early stage of diabetic retinopathy, known as background retinopathy, in which microaneurysms occur due to damage of existing blood vessels. Since the damage occurred in the peripheral retina, rod vision, also called scotopic vision, is affected. There is no obvious damage yet in the macula lutea and the fovea centralis. At these places, cones are found in the greatest number. Since cones are responsible for color vision, choice Ais incorrect. Since the fovea centralis is the place for high acuity vision, choice B is incorrect. Changes in near point vision (choice C) indicate an error in the refractive power of the eye which is associated with abnormalities at the cornea, the lens, or the geometry of the eyeball. Spatial vision (choice E) refers to the ability to discriminate between spatially defined features. It is mainly determined by visual neuronal networks that enhance visual acuity and contrast, which are both associated with photopic vision.

• Question 632:

  A 55-year-old male with a history of smoking presents with a chronic cough with expectoration. He exhibits dyspnea on exertion, has a stocky build, and shows some cyanosis. Which of the following receptors is involved in the cough reflex?

  A. carotid body chemoreceptors

  B. central chemoreceptors

  C. irritant receptors

  D. lung stretch receptors

  E. muscle spindles

  Correct Answer: C

• Question 633:

  Which of the following is an adaptive response to moving from sea level to higher elevation?

  A. bronchial relaxation

  B. decreased cardiac output

  C. decreased circulating levels of EPO

  D. decreased levels of 2,3-diphosphoglycerate (2,3-DPG) in erythrocytes

  E. hyperventilation

  Correct Answer: E

• Question 634:

  A 20-year-old female tennis player has just won a tennis match on a warm summer day. Her blood pressure at this time is 135/70 with a heart rate of 140 beats per minute and a respiratory rate of 25 per minute. She is flushed and sweating profusely. Compared to the resting state, what can be said about the level of activity of sympathetic nerves to her heart and to her cutaneous vasculature?

  A. both are increased

  B. both are decreased

  C. neither is different from at rest

  D. sympathetic activity to the heart is decreased while that to the cutaneous vasculature is increased

  E. sympathetic activity to the heart is increased while that to the cutaneous vasculature is decreased

  Correct Answer: E

  Section: Physiology During exercise in the heat, blood flow must increase to the actively contracting muscles, as well as the cutaneous vessels to shunt heat from the interior of the body to the skin at the surface of the body. Activation of sweat glands allows cooling via evaporation from the surface of the skin. To increase blood flow to the muscles and the skin, it is necessary to increase sympathetic tone to the heart to increase cardiac output. Most sweat glands are activated by sympathetic cholinergic nerves that are activated during sweating. However, the cutaneous vasculature involved in bringing warm blood to the surface of the skin for cooling is constricted by a strong sympathetic tone at rest. During exercise, when body cooling is necessary, sympathetic tone to these cutaneous capillary loops is reduced, causing vasodilation and increased cutaneous blood flow, thus choice E is appropriate.

• Question 635:

  Which of the following statements concerning skeletal muscle is correct?

  A. active tension depends on the extent of overlap between thick and thin filaments

  B. during contraction, thin filaments shorten and thick filaments maintain constant length

  C. significant passive tension is generated at normal rest fiber length

  D. tension generation depends on myosin attachment to the z line

  E. velocity of shortening is independent of fiber load

  Correct Answer: A

  Section: Physiology Tension generation depends on attachment of the myosin head group to actin, this is referred to as cross bridge formation. The amount of tension that can be generated in response to a stimulus depends, in part, on the number of cross bridge attachments that can be formed. Since myosin molecules form the thick filament and actin comprises the thin filament as overlap between thick and thin filaments increases more myosin head groups can bind to actin and more cross bridges can form. Because of this, active tension generation depends on the extent of overlap of thick and thin filaments. Because skeletal muscle is quite compliant, very little passive tension is encountered until the muscle is stretched almost to its maximal length (choice C). During contraction neither the thick nor the thin filaments shortens (choice B). Contraction occurs as thick and thin filaments slide past each other. Velocity of shortening decreases with increasing load (choice E). In skeletal muscle, thin filaments attach to the Z line (choice D).

• Question 636:

  A patient with newly diagnosed schizophrenia is given chlorpromazine. It is a drug that has amongst other effects moderate anticholinergic activity. As a consequence, which of the following is an expected side effect of this medication?

  A. bradycardia

  B. decreased GI sphincter tone

  C. dry mouth

  D. emptying of urinary bladder

  E. increased GI motility

  Correct Answer: C

  Section: Physiology Chlorpromazine is an antipsychotic drug with anticholinergic action (i.e., it inhibits the effects of parasympathetic stimulation). Parasympathetic stimulation causes copious secretion by nasal, lacrimal, and parotid glands. Consequently, parasympathetic blockade is leading to a dry mouth. Parasympathetic fibers slow the heart rate and anticholinergic drugs cause tachycardia rather than bradycardia (choice A). Functions of the parasympathetic nervous system include decreasing GI sphincter tone, increasing GI motility, and increasing emptying of the rectum and urinary bladder. Parasympathetic blockade would thus cause increased GI sphincter tone (choice B), urinary retention (choice D), and decreased GI motility (choice E) leading to constipation.

• Question 637:

  A patient presents with fever (her core temperature equals 39°C, with normal core temperature equaling 3637.5°C) as well as an elevated white cell count. Which of the following statements is true regarding the patient's elevated core body temperature?

  A. Bacterial toxins act directly on skeletal muscle to increase muscle contractile activity, therebygenerating heat and elevating core temperature.

  B. Core body temperature now exceeds the hypothalamic set point temperature.

  C. Increased core body temperature is due to increased heat production by leukocytes.

  D. Increased prostaglandins have raised the hypothalamic set point temperature.

  E. The patient will be sweating in an effort to further elevate core temperature.

  Correct Answer: D

  Section: Physiology In fever, cytokines act to increase prostaglandin levels in the hypothalamus. By an undefined mechanism, prostaglandins increase the hypothalamic set point temperature. The body's heat- generating mechanisms are activated to increase core body temperature until it approaches the new set point. Core temperature will approach but not exceed the set point temperature (choice B). Sweating lowers core temperature (choice E). Though bacterial toxins can be important in the etiology of fever, they do not act directly to increase muscle heat production (choice A). Leukocytes generate cytokines, which act centrally to influence the hypothalamus (choice C). The cellular activity that is most important in heat generation is muscle contraction.

• Question 638:

  Apatient presents with long-standing emphysema. Which of the following would you expect to see in this individual?

  A. decreased physiological dead space

  B. increased FEV1/FVC

  C. increased FRC

  D. increased FVC

  E. increased lung elastic recoil

  Correct Answer: C

  Section: Physiology Destruction of lung tissue is a hallmark of emphysema. Due to this loss of elastic tissue, lung elastic recoil is decreased. Decreased elastic recoil (not increased, choice E) shifts the mechanical balance point between chest wall and lung to a higher volume--FRC is, therefore, increased. The barrel chest is characteristic of emphysema. Destruction of alveolar walls impairs gas exchange, causing an increase in physiological dead space (choice A). Tissue loss also causes a loss of radial traction with an increase in airway resistance--FVC is decreased (choice D). Emphysema is one type of chronic obstructive pulmonary disease. A decrease in the FEV1/FVC is characteristic of an obstructive disease (choice B).

• Question 639:

  A 40-year-old woman complains of chronic fatigue, aching muscles, and general weakness. Physical examination reveals a modest weight gain, dry skin, and slow reflexes. Laboratory findings include TSH: >10 mU/L (normal range 0.55 mU/L), free T4: low to normal. With of the following is the most likely explanation?

  A. hyperthyroidism due to autoimmune thyroid disease

  B. hyperthyroidism due to iodine excess

  C. hyperthyroidism secondary to a hypothalamic-pituitary defect

  D. hypothyroidism due to autoimmune thyroid disease

  E. hypothyroidism secondary to a hypothalamic-pituitary defect

  Correct Answer: D

  Section: Physiology The described symptoms are typical for hypothyroidism. Aprimary thyroid gland deficiency leads to low T4 levels and high TSH levels. The most common cause of thyroid gland failure is called autoimmune thyroiditis or Hashimoto thyroiditis. It develops slowly due to persistent inflammation of the thyroid caused by the patient's own immune system. Middle-aged women are most commonly affected. The measurement of elevated TSH levels in blood is of high diagnostic value, since it helps determine even minor degrees of hypothyroidism. Correct diagnosis is critical because treatment usually continues for life, and stopping of the treatment and reevaluating the original diagnosis is often difficult. Opposite to hypothyroidism, hyperthyroidism (choices A, B, and C) presents with symptoms such as weight loss, warm moist skin, nervousness, and trembling hands. Secondary hypothyroidism (choice E) is caused by reduced TSH levels due to hypothalamic or pituitary insufficiency.

• Question 640:

  A 55-year-old male presents with headache and visual field changes. He is six-and-a-half feet tall and has a puffy face. His skin on hands and feet is thickened, and compared to a picture of him at age 30, his nose, ears, and jaw seem larger. His teeth are separated (diastema). He seems to sweat more and complains of bad sleep. Which of the following would provide the greatest therapeutic benefit to this patient?

  A. GHRH

  B. growth hormone

  C. insulin

  D. somatostatin

  E. thyroid hormones

  Correct Answer: D

  Section: Physiology The symptoms are consistent with acromegaly. It is a rare disease resulting from chronic exposure to growth hormone in adulthood and presents with elevated serum growth hormone levels and elevated serum IGF-1 levels. One treatment option for acromegaly is medication with somatostatin-analogues. These synthetic forms have a longer half-life than the normal polypeptide hormone, which is also called somatotropin release inhibiting factor (SRIF). Growth hormone (choice B), growth hormone- stimulating factors such as GHRH (choice A), and thyroid hormones (choice E) are incorrect treatments. Insulin (choice C) increases bone formation and the calcium content of bone and is therefore not part of the treatment. This can easily be remembered since insulin-like growth factor 1 (IGF-1) is an important stimulator of childhood growth.