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

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 491:

  A 38-year-old female patient suffers from pleurisy and requires pleural fluid sampling (thoracentesis). The attending physician asks you to perform the procedure at the midaxillary line on the right side. Which of the following would be the appropriate level to perform the procedure?

  A. above the level of the 7th rib
  B. at the level of the 10th rib
  C. at the level of the 5th rib
  D. below the level of the 10th rib
  E. between the level of the 8th and 10th ribs
  E. between the level of the 8th and 10th ribs

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  Explanation

  Section: Anatomy The needle for thoracentesis should be inserted in the intercostals spaces between the 8th and 10th ribs. Remember that the parietal pleura extends approximately two ribs inferior to the lung: at the midaxillary line, the inferior surface at the lung is at the level of the 8th rib and the parietal pleura at the 10th rib. Above the level of the 7th (choice A) and 5th (choice C) ribs, the needle will injure the lung. At (choice B) or below the level of the 10th rib (choice D), it will injure the liver or other abdominal organs.

• Question 492:

  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?

  A. intermediate filaments
  B. microfilament
  C. microtubule
  D. neurofilament
  E. tonofilaments
  C. microtubule

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  Explanation

  Section: Anatomy 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.

• Question 493:

  The efferent limb of the pupillary light reflex is interrupted along with corticospinal and orticobulbar fibers in which of the following clinical entities?

  A. Broca's aphasia
  B. inferior alternating hemiplegia
  C. middle alternating hemiplegia
  D. superior alternating hemiplegia
  E. Wallenberg syndrome
  D. superior alternating hemiplegia

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  Explanation

  Section: Anatomy Compression of cranial nerve III, in combination with descending corticospinal and corticobulbar fibers, occurs as part of superior alternating hemiplegia. Patients with Broca's aphasia (choice A) typically do not exhibit involvement of the pupillary light reflexes. Inferior alternating hemiplegia (choice B) and middle alternating hemiplegia (choice C) involve cranial nerves XII and VI, respectively, in combination with corticospinal fibers. Wallenberg syndrome (lateral medullary syndrome) (choice E) typically does not include damage to the corticospinal tract.

• Question 494:

  Apatient has glaucoma following cataract surgery, a moderate elevation of intraocular pressure that often follows replacement of the crystalline lens. Which of the following is a rational choice for treatment and the correct mechanism for that drug?

  A. atracurium--increased outflow of aqueous humor
  B. dorzolamide--increased outflow of aqueous humor
  C. latanoprost--decreased synthesis of aqueous humor
  D. pilocarpine--decreased synthesis of aqueous humor
  E. timolol--decreased synthesis of aqueous humor
  E. timolol--decreased synthesis of aqueous humor

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  Explanation

  Section: Pharmacology Beta blockers such as timolol reduce intraocular pressure by reducing synthesis of aqueous by the ciliary epithelium. Atracurium (choice A) is a nondepolarizing neuromuscular blocking agent that is of no value in glaucoma. Dorzolamide (choice B) is a topically active carbonic anhydrase inhibitor that inhibits the synthesis, not the outflow, of aqueous humor. Latanoprost (choice C) is a PGF-2 alpha analog used topically to increase aqueous outflow. Pilocarpine (choice D) is a muscarinic cholinomimetic that increases aqueous outflow.

• Question 495:

  A 48-year-old man with a long history of chronic viral hepatitis secondary to chronic hepatitis B virus (HBV) infection presented with a 2-month history of weight loss. Physical exam showed a cachectic man in mild distress. Scleral icterus and abdominal distension were present. Workup showed the presence of two mass lesions within the liver on ultrasound, which were confirmed on CT scan. Laboratory values revealed the following: AST 161 U/L; ALT U/L 188; total bilirubin 6.8 mg/dL; direct bilirubin 4.9 mg/ dL; alkaline phosphatase 245 U/L; total protein 5.9 g/dL; albumin 2.8 g/dL; HBsAg positive; anti-HBs negative; anti- HCV negative; alpha-fetoprotein 7200 ng/mL. Aneedle biopsy of one of the mass lesions was performed under CT guidance. Which of the following is the most likely diagnosis?

  A. fholangiocarcinoma
  B. focal nodular hyperplasia
  C. hepatocellular carcinoma
  D. liver cell adenoma
  E. metastatic carcinoma
  C. hepatocellular carcinoma

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  Explanation

  Section: Pathology and Path physiology One of the potential consequences of chronic hepatitis B (and C) infection is the development of hepatocellular carcinoma, and the presence of the mass lesions with high levels of alphafetoprotein makes this extremely likely in this case and will be confirmed by the needle biopsy. Cholangiocarcinoma (choice A) is unusual in the United States but has a much higher incidence in Asia in association with infestation by the liver fluke, C. sinensis. Focal nodular hyperplasia (choice B) is a mass lesion of the liver, but is not a true neoplasm. It has a characteristic central scar and microscopically resembles cirrhosis. Liver cell adenoma (choice D) typically occurs in women taking oral contraceptives. Metastatic carcinoma (choice E) is certainly a possibility but given the clinical history of this case plus the high alpha-fetoprotein and the lack of any evidence for a primary malignant tumor elsewhere, hepatocellular carcinoma remains the most probable diagnosis.

• Question 496:

  A professional football player was diving for a touchdown when his face mask was grabbed and wrenched, causing neck hyperextension and rotation to the right. When brought to the sideline, the player complained of a burning sensation radiating down the right upper extremity and neurological examination revealed right lateral weakness of this limb. Movements affected were arm rotation and flexion, elbow flexion, forearm supination, and thumb flexion. The patient is diagnosed with a brachial plexus injury at the level of C6. 29. Which of the following muscles can perform arm and elbow flexion along with forearm supination?

  A. biceps brachii
  B. brachialis
  C. brachioradialis
  D. coracobrachialis
  E. supinator
  A. biceps brachii

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  Explanation

  Section: Anatomy The biceps brachii muscle attaches proximally by its short head to the coracoid process of the scapula and by its long head to the supraglenoid tubercle. Distally it attaches by a strong tendon to the tuberosity of the radius and by an aponeurosis to the ulna. It thus can perform arm and elbow flexion along with forearm supination. The brachialis (choice B) attaches proximally to the anterior aspect of the lower half of the humerus and distally to the coronoid process of the ulna. It can only perform elbow flexion.

  The brachioradialis (choice C) attaches from the lateral supracondylar ridge of the humerus to the base of the styloid process of the radius. Although innervated by the nerve to the extensor compartment, the radial nerve, it performs elbow flexion and forearm pronation. The coracobrachialis (choice D) attaches from the coracoid process of the scapula to the anterior aspect of the upper half of the humerus. It performs arm flexion and weak adduction. The supinator (choice E) attaches proximally to the lateral epicondyle of the humerus and the annular ligament of the radius. Distally, it covers nearly the upper third of the radius and attaches to its lateral anterior aspect. It supinates the forearm, but is a weaker supinator than the biceps brachii.

• Question 497:

  Patients with fungal pneumonias are not isolated from other patients because of which of the following?

  A. These infections are caused by organisms which multiply in lymph nodes.
  B. These infections are difficult to transmit from person to person.
  C. These infections are eliminated quickly by antifungal drugs (ketoconazole).
  D. These infections are naturally resolved in 24 hours.
  E. These infections provoke an effective immune response.
  B. These infections are difficult to transmit from person to person.

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  Explanation

  Section: Microbiology/Immunology In general, with the exception of some superficial infections, mycoses are not readily transmitted from person to person. Thus, patients with fungal pneumonias are not isolated, and hospital personnel are not generally required to wear masks. Fungal pneumonias are not resolved in 24 hours, they can become chronic, and they need long-term treatment with antifungal drugs (amphotericin B) due to the high percent of cases that relapse (choices C and D). Fungi that cause pneumonias are phagocytized by pulmonary macrophages and multiply in them (choice A). Most of the fungal pneumonias occur in immunosuppressed patients who do not mount an effective immune response (choice E).

• Question 498:

  In February, three patients from a nursing home in East Dallas develop an abrupt onset of a febrile upper respiratory tract illness. They have received no immunizations in the last year. Culture of the nasopharynx of one patient shows influenza A virus. To prevent further cases within the next two weeks, which of the following should be administered?

  A. acyclovir
  B. amantadine
  C. hyperimmune serum globulin
  D. influenza A vaccine
  E. ribavirin
  B. amantadine

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  Explanation

  Section: Microbiology/Immunology Influenza viruses are helical viruses with a segmented, negative-sense RNA genome. A, B, and C types are determined by the protein coat of the virus. The infections are characterized by short (12 days) incubation periods and are efficiently transmitted in respiratory secretions. While most RNA viruses grow only in the infected cell cytoplasm, influenza viruses have growth steps in both the cytoplasm and nucleus of the host cell. The patient's humeral immune system is most important in controlling virus growth. The important antibodies are made against the viral hemeagglutinin (early viral attachment) and the NA (release of mature viruses) enzymes. Amantadine (choice B) and rimantadine block viral uncoating and must be given prophylactically. Oseltamivir (Tamiflu) inhibits NA activity and must also be given before viral infection for best results. Acyclovir (choice A) is an antiherpetic drug. Passive immunity (choice C) does not play a significant role in influenza control. Ribavirin (choice E) is used to treat RSV, a paramyxovirus. Active immunity (choice D) is the most effective mechanism of controlling influenza on a population basis and probably would not be as effective as prophylactic antivirals in the nursing home arena.

• Question 499:

  Which of the following statements best describes interferon's suspected mode of action in producing resistance to viral infection?

  A. It alters the permeability of the cell membrane so that viruses cannot enter the cell.
  B. It stimulates a cell-mediated immunity.
  C. It stimulates humoral immunity.
  D. Its action is related to the synthesis of a protein that inhibits mRNA function.
  E. Its direct antiviral action is related to the suppression of messenger RNA formation.
  D. Its action is related to the synthesis of a protein that inhibits mRNA function.

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  Explanation

  Section: Microbiology/Immunology Interferons are produced by most nucleated cells in the body, and inhibit the translation of viral proteins. There are three major types of interferons, that is, alpha, beta, and gamma. Interferons are active against many viruses and are not specific. Virus infection in one cell induces interferon production which moves to an adjacent, uninfected cell and causes an antiviral protein (AVP) to be produced by the second cell. This AVP inhibits viral mRNA activity and thereby inhibits viral protein production. Interferons do not destroy viruses, but probably result in fewer virions that need to be eliminated by the host's immune system. Interferons do not affect host cell membranes (choice A) or directly suppress mRNA function (choice E). Interferons do not directly stimulate either cellular (choice B) or humoral (choice C) immunity to enhance an antiviral effect.

• Question 500:

  Calcitonin decreases blood calcium and boneresorbing activity in which of the following?

  A. osteoblast
  B. osteoclast
  C. osteocyte
  D. osteon
  E. osteoprogenitor cells
  B. osteoclast

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  Explanation

  Section: Anatomy Calcitonin reduces the surface ruffling of osteoclasts and their activity. Osteoclasts are formed by fusion of blood monocyte derivatives and are components of the mononuclear phagocyte system. Under the influence of parathyroid hormone, osteoclasts enlarge their ruffled borders and increase their bone- resorbing activity. Osteoprogenitor cells (choice E), osteoblast (choice A), and osteocyte (choice C) are bone-forming cells. Osteon (choice D) is another name for the haversian system which is the haversian canal, its contents, surrounding lamellae, and osteocytes.