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

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 731:

  Which of the following immunoglobulins is responsible for passing the placental barrier and attacking the Rh+ fetal red blood cells in hemolytic disease of the newborn (erythroblastosis fetalis)?

  A. IgA
  B. IgD
  C. IgE
  D. IgG
  E. IgM
  D. IgG

  ---

  Explanation

  Section: Microbiology/Immunology There are five known classes of immunoglobulins: IgG, IgA, IgM, IgD, and IgE. IgG is the major immunoglobulin that is found in human serum and the only one that has been shown to pass the placental barrier in humans. IgM possesses higher agglutinating and complement-fixing capacity than IgG. IgM has a molecular weight of 900,000. Carbohydrates constitute 711% of the total weight of IgM (choice E). IgD constitutes a minor portion of serum immunoglobulins (1%). It contains higher amounts of carbohydrate (13%) than the other immunoglobulins, but it is an important B-cell receptor. No other biological functions have been described for IgD (choice B). IgE is the immunoglobulin that has been associated with anaphylactic hypersensitivity. IgE has a molecular weight of 190,000 200,000, contains 1112% carbohydrate, and constitutes 0.002% of the total serum immunoglobulin (choice C). IgA is the major immunoglobulin of extracellular secretions. It has a molecular weight of 160,000440,000, has modest agglutinating capacity, and its carbohydrate content is two to three times higher (7.5%) than that of IgG (choice A).

• Question 732:

  There is but a single enzyme-catalyzed reaction in the human body known to generate carbon monoxide (CO) as one of its products. Which of the following enzymes represents the one that catalyzes this CO- producing reaction?

  A. biliverdin reductase
  B. coproporphyrinogen oxidase
  C. heme oxygenase
  D. protoporphyrinogen oxidase
  E. uroporphyrinogen decarboxylase
  C. heme oxygenase

• Question 733:

  Which of the following conditions is a primary indication for the use of esmolol?

  A. arrhythmia associated with thyroid storm
  B. chronic heart failure
  C. chronic hypertension
  D. diabetic nephropathy
  E. familial tremor
  A. arrhythmia associated with thyroid storm

  ---

  Explanation

  Section: Pharmacology Esmolol is a very short-acting, parenteralonly beta-blocking drug. Transient arrhythmias such as those occurring during thyrotoxicosis are frequently responsive to the drug. Because the drug must be given parenterally and has duration of action of only 15 minutes, it is unsuited for any chronic condition (choices BE).

• Question 734:

  A 45-year-oldman is admitted to the hospital for elective gastrointestinal surgery. On the third post- operative day, the patient experiences a fever of 101.5<pdf2txt-u>癋</pdf2txt-u>, and reports having a slight cough, but is otherwise asymptomatic. Chest x-ray demonstrates a small, interstitial infiltrate in the left lower lobe. Three sets of blood cultures (two bottles each) are drawn and sent to the laboratory. Culture results report positive Staphylococcus epidermidis growth from one bottle of the first culture set, and negative growth from the second and third culture sets. Based on this information, which of the following is the best interpretation of these blood culture results?

  A. intermittent bacteremia associated with postsurgical abscess formation
  B. postoperative septicemia secondary to infective endocarditis
  C. postoperative septicemia secondary to pneumonia
  D. postoperative septicemia secondary to surgical manipulation of the gastrointestinal tract
  E. skin flora contamination and not septicemia
  E. skin flora contamination and not septicemia

  ---

  Explanation

  Section: Pathology and Path physiology True positive blood cultures are defined by positivity in multiple cultures in a series, whereas contaminates generally are not found in repeat cultures. For example, finding S. epidermidis (found in normal skin flora) in only one bottle of six in three blood culture sets (as in this patient) most probably represents skin contamination. If, however, S. epidermidis were isolated from all six bottles drawn from a patient suspected of having infectious endocarditis, this would be interpreted as a true positive. Thus, the results do not support a diagnosis of either bacteremia or septicemia (choices A through D).

• Question 735:

  As a hypothetical approach to treating the hyperglycemia associated with Type II diabetes, a drug firm proposes to develop an inhibitor of liver glycogen phosphorylase. What is the biochemical rationale for this approach to inducing hypoglycemia?

  A. Hepatic fatty acid oxidation will decrease leading to reduced energy production needed for gluconeogenesis.
  B. Hepatocytes will have a reduced capacity to store glucose following meals.
  C. Liver glucose output will be reduced early during fasting.
  D. The resultant increase in glycogen storage will inhibit glucose uptake by the liver, leading to increased usage in skeletal muscle.
  E. There will be an increase in hepatic gluconeogenesis.
  C. Liver glucose output will be reduced early during fasting.

  ---

  Explanation

  Section: Biochemistry During early fasting, as the level of glucose in the blood falls, the pancreas releases glucagon into the circulation to counter this drop. The major site of glucagon action is the liver. There it induces the activity of the glycogen phosphorylase leading to an increase in glucose release from glycogen stores. Thus, an inhibition of glycogen phsphorylase would limit the ability of the liver to provide glucose to the blood. Negatively affecting the activity of glycogen phosphorylase would not significantly affect the rate of hepatic fatty acid oxidation (choice A), skeletal muscle glucose usage (choice D), nor hepatic gluconeogenesis (choice E). The liver may have a reduced capacity for de novo storage of glucose following meals (choice B) due to a prior reduction in the release of glucose via the inhibition of glycogen phosphorylase; however, on fasting there would still be a reduction in glucose release.

• Question 736:

  A 27-year-old woman who recently delivered her first child develops an infiltrative mass in the anterior abdominal wall. A biopsy reveals sweeping fascicles of well-differentiated, plump monomorphic fibroblasts that infiltrate surrounding tissues. Based on this information, what is the most likely diagnosis?

  A. desmoid tumor
  B. fibrosarcoma
  C. malignant fibrous histiocytoma
  D. myositis ossificans
  E. nodular fasciitis
  A. desmoid tumor

  ---

  Explanation

  Section: Pathology and Path physiology Desmoid tumors are mesenchymal tumors that are locally aggressive but do not metastasize and have a benign microscopic appearance characterized by well-differentiated fibroblasts. Those tumors arising in the anterior abdominal wall are usually seen in women who are pregnant or recently postpartum. Fibrosarcomas (choice B) are rare malignant mesenchymal tumors that can metastasize and would typically be expected to have a more pleomorphic microscopic appearance (often with a "herringbone pattern") than that seen in desmoid tumors. Malignant fibrous histiocytomas (choice C) are also malignant mesenchymal tumors that would have a pleomorphic microscopic appearance. Myositis ossificans (choice D) is a non-neoplastic lesion that often forms following blunt trauma to muscle in a lower limb. The initial hematoma undergoes organization with calcification and can have a pleomorphic appearance with numerous mitoses, leading to the possibility of a misdiagnosis as a malignancy. Of special note, however, is that the bone in myositis ossificans matures from the periphery to the center, whereas the opposite pattern is seen in a malignant tumor. Nodular fasciitis (choice E) presents as a rapidly growing, potentially painful mass that may occur in association with prior trauma. Its histological appearance can be worrisome and, similar to myositis ossificans, may lead to misdiagnosis as a malignancy (sarcoma) as it is somewhat pleomorphic with immature fibroblasts and many mitoses. However, these lesions are self-limited and excision is curative.

• Question 737:

  Amember of a college fraternity, who had consumed a large amount of alcohol on a dare, is brought to the emergency room with vomiting and severe abdominal pain. Blood tests reveal that he has elevated serum levels of amylase and lipase. Atentative diagnosis of acute hemorrhagic pancreatitis is formulated. Which cells of the pancreas are directly involved in this condition?

  A. alpha cells
  B. beta cells
  C. centroacinar cells
  D. delta cells
  E. F cells
  C. centroacinar cells

  ---

  Explanation

  Section: Anatomy Centroacinar cells are components of the exocrine pancreas and they contain zymogen granules, which are released under the action of cholecystokinin from the duodenum. The zymogen granules contain inactive proenzymes that normally become activated within the duodenum. Premature activation of the pancreatic enzymes, such as trypsin, leads to autodigestion of the centroacinar cells of the pancreatic acini, which secrete these enzymes. This results in acute hemorrhagic pancreatitis which can be caused by excessive alcohol ingestion. All the other choices are cells of the endocrine pancreas, which secrete hormones. Alpha cells (choice A) secrete glucagon whereas beta cells (choice B) produce insulin. Delta cells (choice C) synthesize gastrin and somatostatin. F cells produce pancreatic polypeptide.

• Question 738:

  Which of the following drugs can be used in rheumatoid arthritis with the lowest probable incidence of GI complications?

  A. aspirin
  B. celecoxib
  C. ibuprofen
  D. misoprostol
  E. naproxen
  B. celecoxib

  ---

  Explanation

  Section: Pharmacology A through C and E are NSAIDs. NSAIDs have long been drugs of first choice in arthritis treatment. Their primary mechanism of action in arthritis appears to be inhibition of COX, an enzyme required for the synthesis of inflammatory and other prostaglandins. Two forms of COX are present in the body:

  COX-1, which is required for synthesis of several useful prostaglandins (e.g., PGE1, a cytoprotective agent in the stomach), and COX-2, the isoform responsible for synthesis of prostacyclin as well as most of the damaging prostaglandins. Celecoxib is more selective for COX-2 and thus has a lower incidence of adverse GI effects. The older NSAIDs (choices A, C, and E) inhibit both COX-1 and COX-2 with less selectivity and thus reduce protective prostaglandins, resulting in a high incidence of GI disorders, especially peptic ulceration. Misoprostol (choice D) is a PGE1 analog that is used with NSAIDs to reduce peptic ulceration; unfortunately it causes a high incidence of diarrhea.

• Question 739:

  As the consulting physician to the US Open, you are asked to examine a golfer who complains of increased pain with right wrist flexion and pronation activities. The patient also reports discomfort even when simply shaking hands with someone. Examination reveals also decreased sensation in the territory of the ulnar nerve. Your diagnosis is golfer's elbow, affecting mostly the superficial flexor muscles of the forearm. This group of muscles has a common origin from which of the following bony landmarks?

  A. head of the radius
  B. lateral epicondyle of the humerus
  C. medial epicondyle of the humerus
  D. olecranon process of the ulna
  E. tuberosity of the radius
  C. medial epicondyle of the humerus

  ---

  Explanation

  Section: Anatomy The superficial layer of flexor muscles of the forearm all originate from the medial epicondyle of the humerus. Thus, this condition is also called medial epicondylitis and the most common finding is tenderness with palpation over the anterior aspect of the medial epicondyle. The muscles involved are most often the Pronator Teres, Flexor Carpi Radialis, and Palmaris Longus. The Flexor Digitorum Superficialis and Flexor Carpi Ulnaris may also be affected. There is no muscle attachment to the head of the radius (choice A). The lateral epicondyle of the humerus (choice B) is the attachment point of the common extensor tendon. The olecranon process of the ulna (choice D) is the attachment point for the Triceps Brachii, Flexor Carpi Ulnaris, and Anconeus. The tuberosity of the radius (choice E) receives the distal tendon of the biceps brachii.

• Question 740:

  An 89-year-old man develops a persistent cough with increasing dyspnea. He worked in the shipyards during his 20s, is a lifelong smoker, and has a positive purified protein derivative (PPD) skin test. During the physical examination, a pleural effusion is detected and subsequently drained. Laboratory analysis reveals a specific gravity of 1.013, protein of 1.4 g/dL, and a small number of macrophages. Based upon these laboratory values, which of the following is the most likely diagnosis for this patient?

  A. heart failure, left-sided
  B. heart failure, right-sided
  C. lung infection, bacterial
  D. lung infection, tuberculous
  E. primary malignancy, squamous cell
  F. primary malignancy, mesothelioma
  A. heart failure, left-sided

  ---

  Explanation

  Section: Pathology and Path physiology Pleural effusion that has a specific gravity less than 1.012 and a protein concentration of less than 1.5 g/dL is classified as a transudate and is associated with circulatory problems (increased hydrostatic pressure, decreased oncotic pressure, lymphatic obstruction). Exudates usually have a specific gravity >1.020 and protein concentration higher than 3 g/dL and are associated with inflammatory or neoplastic processes. Based upon the laboratory results, the patient has a transudate which rules out infection (choices C and D) and neoplasms (choices E and F). The persistent cough and dyspnea indicate that the fluid is accumulating in his lungs, which occurs in left-sided heart failure. Apure right- side heart failure would be expected to produce a systemic rather than pulmonary edema.