Normal flora Practice Questions

Question 1

A medical student is reading about a specific type of T cells that plays an important role in immunologic tolerance. Most of these cells develop in the thymus, but some of them also develop in peripheral lymphoid organs. Usually, they are CD4+ cells and also express CD25 molecules. The functions of these cells are dependent on forkhead box P3 (Foxp3). Their function is to block the activation of lymphocytes that could react with self-antigens in a potentially harmful manner. Which of the following interleukins is secreted by these cells?

Accepted Answer

Interleukin-10

Answer

Interleukin-6

Answer

Interleukin-2

Answer

Interleukin-12

Answer

Interleukin-17

Question 2

A 24-year-old man, an information technology professional, gets himself tested for serum immunoglobulin M (IgM) levels because he wants to know more about his immunity. He knows that IgM levels reflect the status of his immunity, based on the internet. Although the laboratory report is normal, he consults a physician. The physician discusses human immunity and its important components. He also tells him that most circulating IgM antibodies in the blood of normal persons are produced by a specific type of B cell, which is present mostly in the peritoneal cavity and in mucosal tissues. He also mentions that these cells are components of innate immunity. Which of the following types of B cells is the physician referring to?

Accepted Answer

B-1 B cells

Answer

Naïve B cells

Answer

Marginal zone B cells

Answer

Follicular B cells

Question 3

A 37-year-old woman with a history of anorectal abscesses complains of pain in the perianal region. Physical examination reveals mild swelling, tenderness, and erythema of the perianal skin. She is prescribed oral ampicillin and asked to return for follow-up. Two days later, the patient presents with a high-grade fever, syncope, and increased swelling. Which of the following would be the most common mechanism of resistance leading to the failure of antibiotic therapy in this patient?

Accepted Answer

Production of beta-lactamase enzyme

Answer

Intrinsic absence of a target site for the drug

Answer

Use of an altered metabolic pathway

Answer

Altered structural target for the drug

Answer

Drug efflux pump

Question 4

An investigator is studying the growth of an organism in different media. The organism is inoculated on a petri dish that contains heated sheep blood, vancomycin, nystatin, trimethoprim, and colistin. The resulting growth medium is incubated at 37°C. Numerous small, white colonies are seen after incubation for 48 hours. This organism is most likely to cause which of the following conditions?

Accepted Answer

Gonorrhea

Answer

Pontiac fever

Answer

Pseudomembranous colitis

Answer

Hemolytic uremic syndrome

Answer

Oral thrush

Question 5

A team of intensivists working in a private intensive care unit (ICU) observe that the clinical efficacy of vancomycin is low, and proven nosocomial infections have increased progressively over the past year. A clinical microbiologist is invited to conduct a bacteriological audit of the ICU. He analyzes the microbiological reports of all patients treated with vancomycin over the last 2 years and takes relevant samples from the ICU for culture and antibiotic sensitivity analysis. The audit concludes that there is an increased incidence of vancomycin-resistant Enterococcus fecalis infections. Which of the following mechanisms best explains the changes that took place in the bacteria?

Accepted Answer

Replacement of the terminal D-Ala in the cell wall peptidoglycan by D-lactate

Answer

Decreased number of porins in the bacterial cell wall leading to decreased intracellular entry of the antibiotic

Answer

Production of an enzyme that hydrolyzes the antibiotic

Answer

Protection of the antibiotic-binding site by Qnr protein

Answer

Increased expression of efflux pumps which extrude the antibiotic from the bacterial cell

Question 6

A 24-year-old woman presents to the ED with symptoms of pelvic inflammatory disease despite being previously treated with azithromycin for chlamydial infection. Based on your clinical understanding about the epidemiology of PID, you decide to obtain a gram stain which shows a gram-negative diplococci. What is the next step in order to confirm the identity of the organism described?

Accepted Answer

Culture in Thayer-Martin media

Answer

Perform an RT-PCR

Answer

Culture in TCBS agar

Answer

Obtain an acid fast stain

Answer

Culture in Bordet-Gengou agar

Question 7

A 61-year-old woman comes to the physician because of a 5-day history of fever, headache, coughing, and thick nasal discharge. She had a sore throat and nasal congestion the week before that had initially improved. Her temperature is 38.1°C (100.6°F). Physical exam shows purulent nasal drainage and tenderness to percussion over the frontal sinuses. The nasal turbinates are erythematous and mildly swollen. Which of the following describes the microbiological properties of the most likely causal organism?

Accepted Answer

Gram-positive, optochin-sensitive, lancet-shaped diplococci

Answer

Gram-positive, anaerobic, non-acid fast branching filamentous bacilli

Answer

Gram-negative, lactose-nonfermenting, blue-green pigment-producing bacilli

Answer

Gram-negative, oxidase-positive, maltose-nonfermenting diplococci

Answer

Gram-positive, coagulase-positive, clustered cocci

Question 8

A 19-year-old female presents with a malodorous vaginal discharge. She notes that the odor is more prominent after sexual intercourse. She is sexually active with one partner and uses barrier contraception. Her past medical history is significant only for community-acquired pneumonia treated with antibiotics 2 months ago. The vital signs were as follows: blood pressure, 110/70 mm Hg; heart rate, 68/min; respiratory rate, 12/min; and temperature, 36.6℃ (97.9℉). The physical examination was normal. On gynecologic examination, the vaginal walls and cervix appeared normal. There was a gray, thin, homogeneous discharge with bubbles. The pH of the discharge was approximately 6.0. Which of the following findings would be expected on further investigation of this patient?

Accepted Answer

Bacteria-coated epithelial cells on wet mount microscopy

Answer

Numerous gram-positive rod-shaped bacteria on Gram stain

Answer

Gram-positive diplococci on Gram stain

Answer

Negative whiff test

Answer

Motile protozoa on wet mount microscopy

Question 9

A 38-year-old, working, first-time mother brings her 9-month-old male infant to the pediatrician for "wounds that simply won't heal" and bleeding gums. She exclaims, "I have been extra careful with him making sure to not let him get dirty, I boil his baby formula for 15 minutes each morning before I leave for work to give to the caregiver, and he has gotten all of his vaccinations." This infant is deficient in a molecule that is also an essential co-factor for which of the following reactions?

Accepted Answer

Conversion of dopamine to norepinephrine

Answer

Conversion of homocysteine to methionine

Answer

Conversion of alpha ketoglutarate to succinyl-CoA

Answer

Conversion of pyruvate to acetyl-CoA

Answer

Conversion of pyruvate to oxaloacetate

Question 10

While testing various strains of Streptococcus pneumoniae, a researcher discovers that a certain strain of this bacteria is unable to cause disease in mice when deposited in their lungs. What physiological test would most likely deviate from normal in this strain of bacteria as opposed to a typical strain?

Accepted Answer

Quellung reaction

Answer

Hemolytic reaction when grown on sheep blood agar

Answer

Bile solubility

Answer

Optochin sensitivity

Answer

Motility

Normal flora

Study Resources

Lessons

Questions

Flashcards

Lessons

Skin microbiome

Oral microbiome

Respiratory tract microbiome

Gastrointestinal microbiome

Urogenital microbiome

Development of microbiome from birth

Microbiome functions and host defense

Dysbiosis and disease associations

Probiotics and prebiotics

Microbiome in antibiotic-associated diarrhea

Fecal microbiota transplantation

Methods for studying the microbiome

Microbiome and immune system interactions

Practice Questions

Flashcards

About Normal flora for USMLE

Lessons in Normal flora

Master Normal flora