USMLE-STEP-1 Online Practice Questions and Answers

Correct Answer: D

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).

Correct Answer: D

Section: Microbiology/Immunology

P. aeruginosa is a gram-negative, oxidasepositive, aerobic rod that produces a greenblue pigment called pyocyanin. This microorganism has been associated frequently with wound infections in burn patients, and it is the second leading cause of burn infections after S. aureus. P. aeruginosa tends to develop resistance to various antibiotics. However, it may respond to ticarcillin, gentamycin, tobramycin, piperacillin, or azlocillin. E. coli (choice A), K. pneumoniae (choice B), P. mirabilis (choice C), and S. marcescens (choice E) may cause urinary or pulmonary tract infections, but are not considered leading causes of burn infections. Furthermore, these bacteria are oxidase negative and do not produce blue green pigments.

Correct Answer: C

Section: Microbiology/Immunology Exfoliation toxin is produced by certain strains of S. aureus belonging to phage group II. This exotoxin divides the epidermis between the stratum granulosum and the stratum spinosum. The end result of exfoliation action is the exfoliation of the skin, known as the staphylococcal scalded skin syndrome. The TSST (choice E) causes multisystem involvement with shock which can be lethal. TSST is a super antigen that releases large concentrations of IL-1, IL-2, and TNF. Erythrogenic toxin (choice B) is produced by S. pyogenes and is associated with scarlet fever. Alpha toxin (choice A) is produced by S. aureus and causes necrosis and hemolysis of blood cells. Staphylococcal enterotoxins A through F (choice D) are responsible for food poisoning.

Correct Answer: C

Section: Microbiology/Immunology Hepatitis virus Ainfection can best be prevented by immunization with the intact inactivated virus. Interferons act only in the early phase of a viral infection, limiting the spread of virus, and are not recommended for long-term protection against viral diseases (choice E). Immunization with a subunit vaccine is used to prevent hepatitis virus B infection, not hepatitis virus Ainfection (choice D). Hepatitis B core antigen (HBcAg) is not effective for vaccination, even against the hepatitis B virus from which it is derived (choice B). Administration of human gamma globulin may provide only passive immunity, which will be of short duration, and thus it is not the best way to prevent hepatitis A viral infection (choice A).

Correct Answer: A

Section: Microbiology/Immunology The non-spore forming gram-positive bacilli are a diverse group of bacteria. While C. diphtherae is the most important pathogen, many corynebacteria and propionibacteria (anerobes) are members of the normal flora of skin and mucus membranes. These bacteria tend to be clubbed or irregularly shaped. C. durum (choice A) is a diphtheroid, a normal flora species that can be opportunistic in the right location, such as hair follicles and sebaceous glands. It may contribute to lesion development or be part of a mixed culture of skin bacteria present in the pustule. E. rhusiopathial (choice B) causes disease in domestic swine. Erysipelas in humans is caused by Group A streptococci. L. monocytogenes (choice C) enters the body through the GI tract after ingestion of contaminated cheese or vegetables. S. aureus (choice D) and

S. epidermidis (choice E) are welldefined gram-positive cocci.

Correct Answer: D

Section: Pathology and Path physiology An ovarian mass raises the possibility of a tumor and vaginal hemorrhage in a postmenopausal woman and suggests an estrogenproducing tumor. One of the first estrogenproducing tumors to consider is granulosa cell tumor. Arrhenoblastoma (Sertoli-Leydig cell tumor) (choice A) secretes androgens, not estrogens. Brenner tumor (choice B) is an uncommon ovarian neoplasm without hormonal activity. Dysgerminoma (choice C) is the female counterpart of the male seminoma and does not produce hormones. Sertoli-Leydig cell tumor (choice E) is a synonym for arrhenoblastoma (choice A). Teratoma (choice F) is also hormonally inactive.

Correct Answer: C

Section: Pathology and Path physiology Individuals with sickle trait are healthy and not anemic. Hemoglobin electrophoresis demonstrates a minor proportion of hemoglobin S and a major proportion of hemoglobin A. Fetal hemoglobin and A2 hemoglobin are usually normal. Sickle trait confers the benefit of protecting erythrocytes from some forms of malarial infection. About 9% of blacks in the United States have sickle trait. In sickle cell disease (choice A), almost all hemoglobin is hemoglobin S. No hemoglobin A is detected and patients have a clinical history of severe anemia. Sickle thalassemia minor (choice B) presents as a chronic microcytic anemia with a major hemoglobin S component and elevated hemoglobin A2. Thalassemia major (choice D) and thalassemia minor (choice E) do not demonstrate hemoglobin S on electrophoresis.

Correct Answer: B

Section: Pharmacology Dexamethasone, a corticosteroid, combines with a cytoplasmic steroid receptor, enters the nucleus, and alters gene expression. The other agents listed (choices A, CE) act on membrane receptors.

Correct Answer: C

Section: Pharmacology The combination of an ACE inhibitor and a diuretic is rational for this patient with hypertension and mild heart failure as both drugs are effective for both conditions. Furthermore, ACE inhibitors have been shown to slow or stop the progression of heart failure and thiazide diuretics have been shown to be among the cheapest and most effective agents for hypertension. If this combination is not sufficiently active to control the heart failure, a loop diuretic mightbe substituted for the thiazide. Dobutamine (choice A) is a parenteral drug for acute failure. Hydralazine (choices B, E) is a vasodilator that causes tachycardia and usually requires concurrent administration of a beta blocker. Use of two diuretics choice D) is probably unnecessary in this early stage of failure and is not the most effective therapy for hypertension.

Correct Answer: E