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

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 521:

  A 24-year-old woman presents with hypertension and hypokalemic metabolic alkalosis. Although these symptoms are normally indicative of hyperaldosteronism, this patient's aldosterone levels are undetectable, and no other mineralocorticoid activity is found. A diagnosis of Liddle syndrome is made on the basis of the signs and symptoms and a family history. Liddle syndrome is caused by a genetic defect leading to excessive expression of the apical sodium channel in the principal cells of the cortical- collecting tubule and excess sodium transport in this part of the nephron. Which of the following agents is the best choice for treatment of the hypertension and hypokalemic metabolic alkalosis in this patient?

  A. amiloride
  B. fludrocortisones
  C. hydrochlorothiazide
  D. lisinopril
  E. spironolactone
  A. amiloride

• Question 522:

  A 17-year-old boy presented to his orthopedist complaining of progressive knee pain for 3 months. Physical examination revealed a swollen, warm, and tender right knee. Serum alkaline phosphatase levels were increased. Right knee x-ray revealed a lytic lesion in the metaphyseal area of the knee extending into the proximal diaphysis, and elevation of the periosteum with new bone production. Microscopically, biopsy demonstrated haphazard osteoid formation, pleomorphic cells with hyperchromatic nuclei, and tumor giant cells. Based on this information, what is the most likely diagnosis?

  A. chondrosarcoma
  B. Ewing sarcoma
  C. giant cell tumor
  D. malignant fibrous histiocytoma
  E. osteosarcoma
  E. osteosarcoma

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  Explanation

  Section: Pathology and Path physiology In a 17-year-old boy, the anatomical location and the x-ray findings are very typical of osteosarcoma. In this case, the tumor has broken through the cortex, elevated the periosteum with reactive periosteal bone formation (the characteristic triangular shadow seen on x-ray is known radiographically as Codman triangle). The microscopic findings of pleomorphic cells with hyperchromatic nuclei, tumor giant cells, and bone formation confirm the diagnosis. Chondrosarcomas (choice A) most typically occur in the age range of 4060 with the most common locations being the pelvic girdle, ribs, shoulder girdle, and to a lesser extent the long bones, vertebrae, and sternum. Microscopically, the lower grade tumors will show chondroid differentiation, whereas a grade 3 tumor may appear as a spindle cell tumor with little chondroid differentiation. Ewing sarcomas (choice B) occur in children and young adults and usually originate in long bones, ribs, pelvis, and vertebrae. Microscopically one sees small, uniform, oval to round cells with hyperchromatic nuclei and little cytoplasm, which may appear clear due to glycogen abundance. Giant cell tumors (choice C) usually occur in the age range of 2040 most commonly originating in distal femur, proximal tibia, distal radius, and proximal humerus, but virtually any bone may be affected. These are usually benign tumors that can, however, be locally aggressive; roughly 410% do metastasize and these must be considered as malignant. Not surprisingly, the microscopic appearance is marked by the presence of many giant cells that have an osteoclast-like, syncytial appearance. Malignant fibrous histiocytomas (choice D) are fibroblastic, collagen-producing bone tumors that may also arise from soft tissues; indeed, they are the most common type of soft tissue sarcoma. They occur more frequently in the middle-aged and elderly but have been reported at all ages. Microscopically, lesions reveal spindle cells arranged in a whorled pattern with variable amounts of collagen deposition; also present may be large, plump cells (resembling histiocytes), tumor giant cells, many mitoses, and a chronic inflammatory infiltrate.

• Question 523:

  It is known that stretch receptors contained in the walls of the atria convey nerve impulses to the brainstem via the vagus nerve. Under normal conditions, these nerve impulses are most likely to occur during ventricular systole. What information is communicated to the brain by these nerve impulses?

  A. arterial muscle contraction
  B. diastolic arterial pressure
  C. filling of the atrium by venous pressure
  D. filling of the ventricle
  E. systolic arterial pressure
  C. filling of the atrium by venous pressure

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  Explanation

  Section: Physiology The atrial stretch receptors are depolarized by stretch of the atria proportional to the magnitude of venous- filling pressure. The frequency of action potentials rises to a maximum at the peak of the atrial "v" wave, which signals maximum atrial filling during ventricular contraction. Although the remaining choices (A, B, D, and E) may be influenced by the filling of the heart by venous pressure, the information conveyed to the brain via the atrial stretch receptors relates directly to the filling pressure of the atria themselves.

• Question 524:

  Interference with the action of angiotensinconverting enzyme (ACE) is an effective means at reducing elevations in blood pressure. The circulating concentration of which of the following hormones would be affected as a consequence of the use of ACE inhibitors?

  A. ACTH
  B. aldosterone
  C. corticotrophin-releasing hormone, CRH
  D. estradiol
  E. gonadotropin-releasing hormone, GnRH
  B. aldosterone

• Question 525:

  A 28-year-old recently divorced man with no significant past medical history presents to the emergency room with progressive lower abdominal pain and cramping over the past 4 days, both of which are relieved by defecation. He has suffered from substantial bloody and mucoid diarrhea during this time. His temperature is 102.8<pdf2txt-u>癋</pdf2txt-u>. Laboratory studies reveal a n elevated white blood cell (WBC) count and a high erythrocyte sedimentation rate. Sigmoidoscopy reveals extensive rectal and sigmoid hyperemia and edema, numerous superficial ulcerations, and small focal mucosal hemorrhages, many of which have suppurative centers. Significant intestinal narrowing is seen in the distal transverse colon. These findings most likely suggest a diagnosis of which of the following?

  A. amebic colitis
  B. collagenous colitis
  C. cytomegalovirus enterocolitis
  D. pseudomembranous colitis
  E. ulcerative colitis
  E. ulcerative colitis

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  Explanation

  Section: Pathology and Path physiology Clinical history and sigmoidoscopy findings are consistent with a diagnosis of ulcerative colitis, an idiopathic inflammatory bowel disease that involves the rectum in virtually all cases and extends proximally in a continuous manner without skip lesions to include variable lengths of the colon. Psychological stress (recent divorce) may serve to precipitate acute attacks of the disease. The significant narrowing seen in the transverse colon in this patient suggests the possible progression to toxic megacolon, a rare complication in severe acute disease that carries a high mortality rate and requires emergency colectomy. Amebic colitis (choice A) is characterized by acute inflammation with multiple areas of enzymatic tissue necrosis leading to submucosal, flask-shaped abscesses throughout the colon. Clinical features include bloody and mucoid diarrhea with low-grade fever. Collagenous colitis (choice B) is a more recently recognized form of colitis characterized clinically by episodic or chronic watery diarrhea that is not severe enough to cause dehydration, and pathologically by the presence of a distinct collagenous band beneath the colonic surface epithelium. Patients very often have a variety of autoimmune and other diseases. Cytomegalovirus enterocolitis (choice C) is a common GI infection in acquired immunodeficiency syndrome (AIDS) patients that infects the entire GI tract and produces severe, chronic diarrhea. Both mucosal epithelial and vascular endothelial cells are infected. Vasculitis may result in focal ischemic necrosis of the intestinal wall. Pseudomembranous colitis (choice D) is caused by a necrolytic enzyme produced by Clostridium difficile bacteria that most often proliferate secondary to a variety of antibiotic therapies; clindamycin is a frequent culprit. Diarrhea is most often watery and profuse. Gross findings include patchy areas of mucosal inflammation with multiple, discrete yellowish plaques constituting the pseudomembrane, which is composed of fibrin, mucin, and necrotic debris.

• Question 526:

  The structure indicated by arrow 1 in following figure is innervated by which of the following?

  A. anterior ethmoidal nerve
  B. greater palatine nerve
  C. lesser palatine nerve
  D. middle superior alveolar nerve
  E. nasopalatine nerve
  D. middle superior alveolar nerve

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  Explanation

  Section: Anatomy Arrow 1 points to the maxillary sinus, which is innervated by the posterior, middle, and anterior superior alveolar nerves. The latter are branches from the infraorbital nerve arising from the maxillary division (V2) of the trigeminal (fifth cranial) nerve. The anterior ethmoidal nerve (choice A) is a branch of the nasociliary nerve from the ophthalmic division (V1) of the trigeminal nerve. The greater palatine nerve (choice B) and lesser palatine nerve (choice C) are branches of the maxillary division of the trigeminal nerve supplying the hard and soft palate, respectively. The nasopalatine nerve (choice E) is derived from the maxillary division of the trigeminal nerve and supplies the nasal septum.

• Question 527:

  Which of the following normally occupy the dark spaces indicated by the arrows in following figure?

  A. blood vessels
  B. differentiating blood cell precursors
  C. osteoblasts
  D. osteoclasts
  E. osteocytes
  E. osteocytes

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  Explanation

  Section: Anatomy The black areas in this section of dried, compact bone are empty spaces normally occupied by cells and soft tissues. The arrows indicate lacunae interspersed among lamellae of osteons (haversian systems). In life, these are occupied by osteocytes. The fine lines radiating from the lacunae are canaliculi, which contain processes of the osteocytes. Blood vessels (choice A) of compact bone course through Volkmann's canals (not shown) and haversian canals (seen here at the center of each osteon).

  Hematopoietic cells (choice B) occur in red marrow located within medullary canals of long bones and the cavities of cancellous bone. Osteoblasts (choice C) are restricted to surfaces of bone at sites of bone apposition. When osteoblasts become entrapped in lacunae as a result of their synthetic activity, they become osteocytes. Osteoclasts (choice D) are large, multinucleated cells found at surfaces of bone at sites that are undergoing absorption.

• Question 528:

  A 60-year-old man reports to the emergency room complaining of shortness of breath and a rapid, irregular heart beat. An ECG shows the pattern recorded in below figure, panel A. Adiagnosis of atrial fibrillation, heart rate 140, is made. He is treated with an IV drug, which results in a change of the ECG, 1 hour after drug administration, to the pattern shown in panel B. This ECG was read as atrial fibrillation, heart rate 65.

  What drug was used?

  

  A. adenosine
  B. diltiazem
  C. disopyramide
  D. ibutilide
  E. quinidine
  B. diltiazem

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  Explanation

  Section: Pharmacology The initial atrial fibrillation ECG shows a very rapid ventricular rate, which decreases cardiac output and explains the patient's shortness of breath. After administration of the drug, the ventricular rate markedly slowed, but QRS duration and QT interval were not altered. This pattern of effects is observed with drugs that selectively slow AV conduction (i.e., beta blockers [antiarrhythmic group 2] and calcium channel blockers [group 4]). Adenosine (choice A) slows or blocks AV conduction but has a very short duration of action (seconds). Disopyramide (choice C) and quinidine (choice E) are group 1Aantiarrhythmic drugs and prolong the QRS and QT intervals. Ibutilide (choice D) is a group 3 drug and prolongs the QT interval. Cardiac glycosides such as digoxin also slow AV conduction (through their parasympathomimetic action), but have a slower onset of action than parenteral calcium channel blockers or beta blockers.

• Question 529:

  Asuburban housewife was pricked by a thorn while pruning her roses. Four days later, a pustule that changed to an ulcer developed in the lesion area. Then three nodules formed along the local lymphatic drainage. The most likely agent is which of the following?

  A. fumigates
  C. Cryptococcus neoformans
  D. Sporothrix schenckii
  E. Trichophyton rubrum
  D. Sporothrix schenckii

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  Explanation

  Section: Microbiology/Immunology

  S. schenckii is found on thorns, and it is introduced into the skin of extremities through trauma. A regional lesion begins as a pustule, abscess, or ulcer, and then nodules and abscesses are formed along the lymphatics. The history and the symptoms described in this patient are consonant with a diagnosis of sporotrichosis.

  A. fumigatus (choice A) and

  C. albicans (choice B) are associated with deep opportunistic infections in immunocompromised patients such as AIDS patients. Aspergillosis is basically a

  pulmonary infection. Candidiasis can be associated with pathological conditions of the mucous membranes of the respiratory, genital, and gastrointestinal tract, where it is found as a normal inhabitant.

  C. neoformans (choice C) is the cause of meningitis.

  T. rubrum (choice E) is the cause of dermatophytosis ringworm of skin, scalp, and especially nails. The nails thicken and are discolored.

• Question 530:

  Which of the following substances contains the highest concentration of molecules with estrogenic effects?

  A. ginseng
  B. grapefruit juice
  C. kava
  D. saw palmetto
  E. tomatoes
  D. saw palmetto

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  Explanation

  Section: Pharmacology Saw palmetto is promoted by purveyors of herbs and "alternative medicines" as therapy for prostatic hyperplasia and does have weak estrogenic activity. Ginseng (choice A) is known as an adaptogen, which means it increases resistance to physical, chemical, and biological stress and builds up energy and general vitality, including the mental and physical capacity for work. Grapefruit juice (choice B) and tomatoes (choice E) are used therapeutically in a treatment regimen referred to as "raw juice therapy." Raw juices of fruits and vegetables are extremely rich in vitamins, minerals, trace elements, enzymes, and natural sugars. Kava (choice C) is used in herbal therapy to treat anxiety, nervousness, nsomnia, stress, and restlessness. Only saw palmetto is known to have any estrogenic activity.