USMLE USMLE-STEP-1 Online Practice Questions and Exam Preparation

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 771:

  Darrin is an intern working in the emergency room at the local county hospital. Mr Arnold, a 48-year- old homeless man, comes in complaining of "feeling real bad." Mr Arnold's mentation is altered, and is unable to verbalize his past medical history. Darrin is having difficulty examining the patient because his body emits a tremendous odor. While examining him, Darrin discovers that his right leg is red, swollen, sore, and covered with ulcers. The nurse in the emergency room covers the leg with dressings and a plastic bag to contain the smell. By morning, Mr Arnold's anaerobic blood cultures become positive with grampositive rods. Darrin observes that the bag is full of air, and when he removes it the smell is overwhelming. What should Darrin do?

  A. Arrange for Mr Arnold to receive hyperbaric oxygen therapy.
  B. Consult with a surgeon, as Mr Arnold will need debridement and IV antibiotics.
  C. Continue the broad spectrum IV antibiotics he has prescribed for Mr Arnold.
  D. Put Mr Arnold in respiratory isolation and confirm his diagnosis with a spinal tap.
  B. Consult with a surgeon, as Mr Arnold will need debridement and IV antibiotics.

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  Explanation

  Section: Microbiology/Immunology Darren's patient is most likely suffering from an infection that is caused by toxin-producing clostridia. C. perfringens and related clostridia can produce invasive infection (myonecrosis and gas gangrene) if they are introduced into damaged tissue. These organisms produce a large variety of toxins that may be lethal, necrotizing, and/or hemolytic. Clostridia are anerobic bacteria and can only grow well in conditions of low oxygen contact, such as necrotic tissue. In such an infection, within a few days there is foul-smelling discharge, fever, hemolysis, toxemia, shock, and death. Treatment includes immediate surgery and antibiotic administration. Hence, choice B would be the next appropriate medical intervention. Hyperbaric oxygen (choice A) has no proved effect in this situation. Antibiotics only (choice C) would provide only partial resolution and not in a timely manner. Choice D has nothing to do with the medical management of Darren's patient.

• Question 772:

  In elderly patients (over 60 years of age), fractures of the neck of the femur following a fall are common. Arterial branches supplying the femoral head and neck are vulnerable to injury during these fractures, and the resulting posttraumatic avascular necrosis affects the head of the femur. In the adult, the most important direct vascular source to the femoral head and neck is which of the following?

  A. artery to the head of the femur
  B. femoral artery
  C. lateral circumflex femoral artery
  D. medial circumflex femoral artery
  E. superior gluteal artery
  D. medial circumflex femoral artery

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  Explanation

  Section: Anatomy The medial circumflex femoral artery supplies the most important source of blood to the femoral head and neck. This artery anastomoses with the artery to the head of the femur (choice A), which arises from the obturator artery. However, if the medial circumflex femoral artery is injured, the blood flow in the small artery to the head of the femur may not be sufficient to prevent posttraumatic avascular necrosis of the femoral head. Normally, the medial and lateral circumflex femoral arteries arise from the deep artery of the thigh, but occasionally they arise from the femoral artery (choice B). However, the femoral artery is not a direct vascular source to the head of the femur. The lateral circumflex femoral artery (choice C) and superior gluteal artery (choice E) also supply the hip joint, but their contribution to the head and neck of the femur is less than that of the medial femoral circumflex artery.

• Question 773:

  A 31-year-old previously healthy woman develops pelvic inflammatory disease and subsequently undergoes a complete hysterectomy; her ovaries are left intact. Incidental findings in the myometrium are three discrete, sharply circumscribed masses that range from 4 to 8 cm in diameter and have a whitish, whorled cut surface. What would have been the most likely outcome over the next 5 years if these masses had not been removed?

  A. complete regression
  B. continued expansile growth
  C. infiltration into surrounding tissues
  D. metastasis via blood vessels
  E. necrosis and hemorrhage
  B. continued expansile growth

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  Explanation

  Section: Pathology and Path physiology This description of the myometrial masses is consistent with leiomyomas, benign tumors of smooth muscle origin. Uterine leiomyomas are the most frequent tumor in women and are estimated to occur in up to 75% of women of reproductive age. As benign tumors, the expected outcome for such masses in a premenopausal woman would be continued expansile growth. Growth of leiomyomas is stimulated (but not initiated by) estrogen, therefore, regression (choice A) would be more probable in a woman of postmenopausal age or a younger woman who may have had her ovaries removed but is unlikely to be complete. Infiltration into surrounding tissues (choice C) and metastasis via blood vessels (choice D) are characteristics of malignant tumors only. While necrosis and hemorrhage (choice E) may rarely occur in benign neoplasms, they are quite characteristic of malignant neoplasms, as blood vessels are more numerous and are often poorly formed. Uterine leiomyomas and colonic adenomas are two of the exceptions to the general rule that benign neoplasms develop as single lesions.

• Question 774:

  The electrocardiogram of a 66-year-old male with a history of atherosclerotic heart disease reveals an irregular, but rapid heart rate. The QRS complexes are normal but no P-waves can be seen. What is the most likely reason for these findings?

  A. atrial fibrillation
  B. Paroxysmal ventricular tachycardia
  C. Right bundle branch block
  D. Sinus tachycardia
  E. Wolff-Parkinson-White syndrome
  A. atrial fibrillation

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  Explanation

  Section: Physiology Atrial fibrillation is a continuous, chaotic reentry of electrical impulses within the atrial myocardium that arises in a diseased or stretched left atrium. The chaotic patterns of atrial excitation prevent P- waves from being seen in the electrocardiogram. The ventricular response is rapid and irregular. In all the other alternatives (choices BE), a P-wave should be distinguishable in some part of the electrocardiogram, although not necessary in a constant relationship to the QRS complex.

• Question 775:

  Which of the following correctly describes Norwalk virus?

  A. contains an RNA genome with a single open reading frame
  B. generally causes a severe chronic gastroenteritis with diarrhea that may persist for several weeks
  C. is a major cause of severe diarrhea in infants in the developing countries
  D. is an important cause of epidemic gastroenteritis in the United States
  E. is diagnosed by isolation and then growth of the virus in tissue culture
  D. is an important cause of epidemic gastroenteritis in the United States

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  Explanation

  Section: Microbiology/Immunology Many outbreaks of intestinal disease and diarrhea occur in crowded conditions like cruise ships, where hundreds of tourists become ill in a short time and the cruise must be terminated early so that treatment may be obtained and the ship sanitized. Norwalk virus belongs to the calicivirus classification and is an important cause of outbreaks of vomiting and diarrheal illness in families, communities, and institutions. It is widespread in the United States and is the most important cause of epidemic viral gastroenteritis in adults (choice D). Caliciviruses do have a single-stranded RNA genome which is positive-sense and nonsegmented. It has multiple open reading frames for translation (choice A). The gastroenteritis caused by Norwalk virus has a 2448 hour incubation, rapid onset, and a brief time course (1260 hours), not several weeks (choice B). Rotaviruses are the major cause of severe diarrhea in infants in developing and developed countries (choice C). Reverse transcriptase-- PCR is the most widely used method to diagnose Norwalk and other caliciviruses in outbreaks since the virus cannot be isolated, grown, and identified in diagnostic laboratories (choice E).

• Question 776:

  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
  E. water intoxication

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  Explanation

  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 777:

  In a quantitative serological test, the results shown below were obtained (+ = antibody detected, 0 = no antibody detected). Which of the following courses of action should be taken?

  

  A. choose another test mechanism to measure the antibody
  B. perform testing on additional dilutions until an end point is reached
  C. repeat the test because this pattern is impossible
  D. report "negative" for antibody E) report "positive" for antibody
  B. perform testing on additional dilutions until an end point is reached

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  Explanation

  Section: Microbiology/Immunology Quantitative serological tests are being replaced by newer testing procedures, such as ELISA, which is not quantitative. Understanding how quantitative data is used diagnostically is still important clinically. For best results for interpretation, an acute and convalescent serum from thepatient should be tested at the same time. If a 4-fold (1:2 dilutions) or 100-fold (1:10 dilutions) increase in Ab titer is found, that would be a positive diagnostic result. Often, as in this case, only a single serum sample was tested.

  Positive results through the highest dilution of serum could represent a very high Ab level or some problem with the test procedure, giving a false-positive interpretation. The most effective way to solve this problem is to dilute the serum further and repeat the test (choice B). Once an end-point is reached, this information may confirm or deny a possible diagnosis. An alternative test mechanism (choice A) may be available, but one must remember not to use results from two test mechanisms as a direct comparison for diagnostic purposes. This pattern is not uncommon (choice C) because high antibody titers may exist in some infections (EBV, for example). Negative (choice D) or positive (choice E) would be inappropriate reports without repeating the test as described above.

• Question 778:

  A 39-year-old man with normal sexual practices saw his doctor, complaining of a mild (38.5°C) but persistent fever. His physician observed pharyngitis and a bilateral conjunctivitis that the patient claimed was recent and painful. Which of the following microbes most likely caused the infection?

  A. adenovirus
  C. N. gonorrhoeae
  D. S. pyogenes
  E. Toxoplasma gondii
  A. adenovirus

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  Explanation

  Section: Microbiology/Immunology Adenoviruses are a frequent cause of infection characterized by a triad of symptoms, namely, pharyngitis, conjunctivitis, and fever. S. pyogenes (choice D) causes 25% of cases of pharyngitis. However, the disease is often accompanied by tonsillitis. The infection is usually associated with fever which is higher than 38.5°C. N. gonorrhoeae (choice C) normally causes gonorrhea. C. pneumoniae (choice B) causes generally primary atypical pneumonia which is not usually associated with conjunctivitis. T.

  gondii (choice E) in AIDS patients, or other immunosuppressed individuals can cause disseminated disease and encephalitis (choice E).

• Question 779:

  A high school student while on a field trip encounters poison ivy and develops a severe contact dermatitis, which is classified as type IV hypersensitivity. Contact dermatitis is differentiated from type III hypersensitivity by which of the following facts?

  A. It is associated with cell bound antigens of human erythrocytes.
  B. It is associated with cytotoxic reactions.
  C. It is encountered in atopic individuals.
  D. It is mediated by antigen-antibody complexes.
  E. It is mediated by cellular immunity.
  E. It is mediated by cellular immunity.

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  Explanation

  Section: Microbiology/Immunology Type IV hypersensitivity, such as contact dermatitis, is a delayed-type allergic reaction that is differentiated from immediate type III hypersensitivity by the fact that it is mediated by a T-cell response. Type III, II, or I allergic reactions are mediated by humoral antibody. Atopic individuals exhibit type I hypersensitivity (choice C). Type II allergic reactions involve production of antibody (IgG, IgM) to cell-bound antigens of erythrocytes. Transfusion reactions are good examples of type II hypersensitivity (choice A).

  Cytotoxic reactions are type II allergic reactions (choice B). Immune complex reactions represent type III hypersensitivity. Here antigen-antibody (IgG or IgM) complexes accumulate on blood vessels or kidney glomeruli and cause tissue damage. Examples of type III allergy is the Arthus reaction or serum sickness (choice D).

• Question 780:

  In emphysema, which of the following components of the bronchioles is affected?

  A. ciliated cuboidal epithelial cells
  B. Clara cells
  C. elastic fibers
  D. goblet cells
  E. squamous type I alveolar epithelial cells
  C. elastic fibers

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  Explanation

  Section: Anatomy Elastic fibers are destroyed in emphysema by elastase. This protease is released by neutrophils recruited by macrophages under abnormal stimulus such as cigarette smoke. The loss of elasticity in the bronchioles and alveolar walls gives rise to emphysema, characterized by chronic airway obstruction. Ciliated cuboidal epithelial (choice A) and Clara (choice B) cells line the terminal bronchioles. Goblet cells (choice D) may be found at the beginning of the bronchioles and squamous type I alveolar epithelial cells line the respiratory bronchioles.