Micro USMLE 8-20 (2)

Question 1

Individuals with acute bacterial endocarditis typically present with fever, chills, and other systemic symptoms; and in those with a history of?

Answer 1

IV drug use, tricuspid valve involvement is common. Pleuritic chest pain occurs when septic emboli from the tricuspid valve lodge in the pulmonary circulation.

Question 2

The combination of an elevated temperature, a lytic lesion in the bone, and symptoms of gait instability form a typical presentation of osteomyelitis in a child. The most common cause of osteomyelitis in otherwise healthy patients is ?

Answer 2

Staphylococcus aureus, which accounts for ≥50% of cases. Hematogenous spread is the most common cause of osteomyelitis in children. Bacteria most commonly seed the metaphysis due to the rich, slow-flowing blood supply.
S. aureus is a gram-positive organism that is typically seen in clusters. In addition to causing osteomyelitis, the organism causes toxic shock syndrome, staphylococcal scalded skin syndrome, folliculitis, food poisoning, and sepsis. S. aureus is catalase+ and coagulase+.

Question 3

Gram-negative bacilli in clusters, such as E. Coli and Salmonella, tend not to cause osteomyelitis in otherwise healthy children, though Salmonella are a cause of osteomyelitis in patients with sickle cell. Non- S. aureus staphylococcus species (gram-positive cocci that are catalase+ and coagulase-) are a rare cause of osteomyelitis in people with native joints.

Streptococcus pneumoniae is a gram-positive, catalase - organism that is a rare cause of ?

Answer 3

osteomyelitis. Finally, Neisseria species are gram negative cocci in pairs, and can cause septic joints but not usually osteomyelitis, and not usually in children.

Question 4

A 56-year-old woman presents to her primary care physician with a painful cut on her lower leg that she sustained when she scraped it on a rusty car door. On physical examination, her leg is warm and erythematous, with white purulent material oozing from the wound site. Laboratory evaluation of the purulent material shows gram-positive organisms distributed in clumps. Laboratory studies show methicillin susceptibility.

Which of the following is the best first-line treatment for this patient?

Answer 4

Dicloxacillin

Staphylococcus aureus infections can have many different manifestations, including skin infections, osteomyelitis, abscesses, bacteremia, toxic shock syndrome, and pneumonia. Most S. aureus strains are susceptible to methicillin, dicloxacillin, oxacillin, and nafcillin which are penicillinase-resistant penicillins, as well as first-generation cephalosporins, making these drugs the first-line treatment options. Historically, methicillin was the drug of choice; however, it has since been discontinued due to its nephrotoxicity, and now dicloxacillin, oxacillin, and nafcillin or a first-generation cephalosporin are used in its place. Infection with strains of methicillin-resistant S. aureus would require the use of other drugs such as vancomycin.

Question 5

Gentamicin is used to treat gram-negative microbial infections but is not effective for the treatment of Staphylococcus aureus. Gentamicin belongs to the aminoglycoside group of antibiotics.

Penicillin G is an intravenous antibiotic that is bactericidal for gram-positive and some gram-negative organisms. Because most strains of Staphylococcus aureus have penicillinase enzyme, they are typically not susceptible to penicillin.

Piperacillin-tazobactam is used to treat?

Answer 5

Pseudomonas infections, resistant Staphylococcus aureus, and many gram-negative infections. It would not be a first-line therapy for S. aureus infection.

Vancomycin is effective for the treatment of gram-positive organisms, including Staphylococcus aureus. It is not indicated, however, unless the strain is methicillin-resistant or the patient has a penicillin allergy.

Question 6

Staphylococcus aureus is involved in many skin infections, and is a gram-positive organism that appears in clusters on Gram stain. It contains penicillinase, and so requires? ).

Answer 6

a penicillinase-resistant antibiotic such as oral dicloxacillin or intravenous nafcillin/oxacillin for first-line treatment (as long as it is methicillin-sensitive

Question 7

This patient presents with a multilobar pneumonia caused by Staphylococcus aureus. The radiographic findings describe a pneumatocoele, an air-filled cavity most often seen in S. aureus infections (not to be confused with an abscess which will contain purulent material). Nafcillin is a β-lactam antibiotic with excellent coverage of gram-positive cocci. It does not cover methicillin-resistant or vancomycin-resistant organisms. S. aureus can affect many tissue types and most commonly causes?

Answer 7

skin and soft tissue infections but can also cause more invasive infections such as pneumonia, endocarditis, and osteomyelitis. Risk factors for S. aureus infection include indwelling catheters, preceding respiratory infection (eg, influenza), cystic fibrosis, and surgical wounds.

Question 8

Nafcillin is a β-lactam antibiotic with excellent coverage against gram-positive cocci and is effective in treating ?

Answer 8

Staphylococcus aureus pneumonia.

Question 9

Bacteroides fragilis is an anaerobic bacterium that is found in a variety of infections but is especially common in abdominal infections. It can lead to peritonitis or intraperitoneal abscesses. Treatment of anaerobic infection involves use of metronidazole or clindamycin. Abscesses may require surgical drainage.

Actinomyces species can be identified as?

Answer 9

gram-positive rods with branching filaments. Actinomyces infection usually involves the cervical or facial region and is associated with abscesses, sinus tract infections, and fistulas. It is easily confused with other diseases of the head and neck, such as malignancy. Antibiotic treatment of Actinomyces species infections is with penicillin G or with tetracyclines in the case of penicillin allergy.

Question 10

Bacillus anthracis is a gram-positive, aerobic, spore-forming, rod-shaped bacterium. Infection can be of either the cutaneous or the inhalational form. The cutaneous form is transmitted by contact with spores and is manifested by cutaneous ulceration and eschar formation. Treatment for cutaneous infection is with penicillin, erythromycin, or ciprofloxacin. The inhalational variety, also called woolsorters’ disease, presents with nonspecific symptoms of fever and malaise but progresses to respiratory failure. Treatment of the inhalational infection is with intravenous penicillin, although most patients will die despite treatment.

Pseudomonas aeruginosa is an oxidase-positive, non-lactose-fermenting, gram-negative, rod-shaped bacterium that causes ?

Answer 10

many different types of infection, including pneumonia, swimmer’s ear, urinary tract infection, and hot-tub folliculitis. It is an aerobic gram-negative rod that produces pyocyanin, a blue-green pigment. P. aeruginosa infection can be treated with many agents, including antipseudomonal penicillins, ciprofloxacin, and antipseudomonal cephalosporins such as fourth-generation cephalosporins like cefipime.

Question 11

The sinus tracts on the buccal mucosa, the yellow exudate, and the mandibular lesion suggest that this patient has an oral abscess caused by the gram-positive organism Actinomyces israelii. A. israelii is an anaerobic bacillus that forms long branching filaments that resemble fungi but are much thinner by comparison. This organism is part of the normal flora of the mouth, colon, and vagina and tends to cause infection in patients with dental caries, extractions, or gingivitis/gingival trauma. Men with poor oral hygiene are at highest risk. The characteristic feature of this organism is ?

Answer 11

the yellow clumps, known as sulfur granules. Treatment of A. israelii infection is intravenous penicillin for 2-6 weeks, followed by oral therapy with penicillin or amoxicillin for 6-12 months.

Question 12

Nocardia asteroides is a gram-positive, weakly acid-fast organism that grows in a filamentous pattern similar to some fungi. It is often confused with Actinomyces. However, N. asteroides mainly causes pulmonary infection in immunocompromised patients.

Pseudomonas aeruginosa is an oxidase-positive, gram-negative bacillus that causes skin infection in burn victims and pneumonia in those with cystic fibrosis. This bacterium produces a blue-green pigment.

Serratia marcescens is a member of the Enterobacter family, which is notable for the production of a bright red pigment. It is a common cause of?

Answer 12

urinary tract infections, wound infections, or pneumonia. While it could infect the type of wound described in this question, it would not produce yellow granules.

Staphylococcus aureus is a catalase-positive, coagulase-positive, gram-positive coccus that is the most common cause of skin and soft tissue infections. Although these bacteria produce gold-colored colonies when cultured, they do not form the characteristic sulfur granules of actinomycotic infections.

Question 13

Branching rods on culture from a patient with an oral infection are likely?

Answer 13

Actinomyces israelii.

Question 14

This patient’s symptoms of fever, desquamating rash, and hypotension in the setting of retained nasal packing are consistent with toxic shock syndrome caused by Staphylococcus aureus. S aureus produces a toxin called toxic shock syndrome toxin (TSST-1) that acts as a superantigen. TSST-1 cross-links ?

Answer 14

major histocompatibility complex II (MHC II) molecules on antigen-presenting cells to T-cell receptors on T cells independent of antigen. This results in T-cell proliferation, which can induce a cytokine storm. The presence of these cytokines (ie, interleukin-1, interleukin-2, interferon-γ, and tumor necrosis factor-α) gives rise to fever, hypotension, and a diffuse macular rash (like that shown in the image), which desquamates after a few days. Retained foreign bodies that can serve as a conduit for infection, such as nasal packing or tampons, make S aureus the most likely culprit, as this organism naturally colonizes the nose and skin.

Question 15

Toxic shock syndrome is mediated by inappropriate T-cell activation by TSST-1. This mechanism differs from molecular mimicry (binding of antibodies that recognize a similar epitope on normal tissue), which is classically associated with rheumatic fever and occurs 2 to 3 weeks after infection (typically with Streptococcus pyogenes).

Although this patient’s inflammatory response is mediated by cytokines, it is not due to?

Answer 15

excess cytokine release by B-cell activation. B cells do not secrete the constellation of cytokines that cause the pathology described.

Question 16

A type 1 hypersensitivity reaction, cross-linking of IgE on presensitized mast cells and basophils, triggering release of their contents, is seen with anaphylaxis (eg, after a bee sting) or an atopic condition (eg, eczema) and causes a rash that occurs within minutes and is accompanied by urticaria, edema, and pruritus.

Although Streptococcus pyogenes can mediate a toxic shock–like syndrome due to nonspecific T-cell activation by exotoxin A, the patient&’s history of retained nasal packing makes?

Answer 16

S aureus and TSST-1 exotoxin more likely. Streptococcal pyrogenic exotoxin A has identical superantigen activity and could be a plausible cause if the patient had a history of prior streptococcal skin infection (eg, cellulitis).

Question 17

Staphylococcus aureus produces a superantigen, toxic shock syndrome toxin (TSST-1), which activates a large population of T lymphocytes by cross-linking MHC II and T-cell receptors. This results in a clinical triad of ?

Answer 17

fever, shock, and a desquamating rash in the setting of a retained foreign body, such as nasal packing or a tampon, which provides a rich nutrient source (blood) for the bacteria.

Question 18

This patient’s clinical history includes a recent mechanical fall; the presence of a single, swollen, tender, and erythematous joint; and cloudy, yellow joint aspirate with gram-positive cocci in clusters. This presentation is most suggestive of septic arthritis. Septic arthritis typically presents as monoarticular pain in a joint that is swollen, red, and painful. Synovial fluid is typically purulent with a white blood cell count ≥50,000/mm3. The most common cause of septic arthritis in adults and children older than 2 years is ?

Answer 18

Staphylococcus aureus. The Gram stain illustrating gram-positive cocci in clusters also supports S aureus as the most likely causative organism of this patient’s septic arthritis.
Virulence factors help bacteria evade the host’s immune response. S aureus is a coagulase-positive, catalase-positive organism with the virulence factor protein A. This protein binds the Fc portion of immunoglobulin G (IgG), which prevents opsonization and phagocytosis.

Question 19

Other bacteria that can cause septic arthritis include Neisseria gonorrhoeae, viridans streptococci, Streptococcus pneumoniae, and group B streptococci. Gonorrhea is common in young (≤ 35 years old), sexually active patients. Streptococcus species are associated with septic arthritis in patients who are ?

Answer 19

immunocompromised (eg, those receiving immunosuppressive or steroid therapy and those with diabetes), intravenous drug users, and the very young (ie, ≤2 years old) or old (ie, ≥65 years old).

Other considerations for this presentation of tender joint pain include reactive arthritis. Reactive arthritis is typically seen after a gastrointestinal or genitourinary infection (such as with Escherichia coli or Vibrio cholerae) and would not be consistent with the patient’s denial of nausea, vomiting, or diarrhea.

Question 20

M protein is the virulence factor for Streptococcus pyogenes, which can cause rheumatic fever.
Immunoglobulin A protease is the virulence factor for Streptococcus pneumoniae and Neisseria gonorrhoeae.
Fimbriae and pili are the virulence factors for Neisseria meningitidis, Escherichia coli, and Vibrio cholerae.
Exotoxin A is produced by?

Answer 20

group A streptococci and causes toxic shock–like syndrome.
Lipid A is a virulence factor for gram-negative bacteria. Lipid A is a structural component of lipopolysaccharides, which compose the cell walls of gram-negative bacteria.

Question 21

The most common cause of impetigo is Staphylococcus aureus, followed by Streptococcus pyogenes. S. aureus is typically ?

Answer 21

coagulase positive, whereas other Staphylococcus species are coagulase negative. Nonbullous impetigo is characterized by an eruption of vesicles on the face. These vesicles later turn into pustules with a characteristic honey-colored crust. Bullous impetigo is characterized by fluid-filled bullae that also rupture, leaving a thin brown crust (see image). Bullous impetigo is caused by strains of S. aureus that produces exfoliative toxin A, which disrupts cell adhesion in the epidermis.

Question 22

The presence of a lipopolysaccharide endotoxin in the cell membrane is a characteristic of gram-negative bacteria, but not Staphylococcus aureus or Streptoccus pyogenes. Endotoxin triggers multiple reactions, including activating macrophages, the complement pathway, and possibly coagulation cascades.

Sabouraud agar is required to culture fungi, not Staphylococcus aureus. Other important agars include Lowenstein-Jensen agar for Mycobacterium tuberculosis, charcoal yeast extract agar with cysteine for Legionella, chocolate agar for Haemophilus influenzae, and Bordet-Gengou agar for Bordetella pertussis.

Streptococcus pneumonia and the Viridans group Streptococcus are both α-hemolytic, but Staphylococcus aureus and Streptococcus pyogenes are?

Answer 22

β-hemolytic.

The second most common cause of impetigo is Streptococcus pyogenes, a gram-positive group A β-hemolytic organism that is bacitracin sensitive. Bacitracin is used to differentiate S. pyogenes (bacitracin sensitive) from S. agalactiae (bacitracin resistant), whereas optochin is used to differentiate S. pneumoniae (optochin sensitive) from viridans group streptococci (optochin resistant).

Staphylococcus aureus is a gram-positive cocci that is a common cause of impetigo. The next most common cause is Streptococcus pyogenes, either alone or in conjunction with S. aureus. Nonbullous impetigo is characterized by vesicles that become pustules and develop a yellow crust.

Question 23

This patient’s diffuse desquamating rash, hypotension, fever, and warm extremities are sugggestive of shock. Given her history of retention of a tampon for the last 3 days, she is likely experiencing staphylococcal toxic shock syndrome (TSS). Staphylococcus aureus releases TSS toxin 1 (TSST-1), which is a superantigen. TSST-1 cross-links major histocompatibility complex II molecules with certain T-lymphocyte receptor subsets, leading to ?

Answer 23

excessive T-lymphocyte activation. This leads to supraphysiologic production of cytokines, including interleukin-1 (IL-1), IL-2, IL-6, interferon-γ, and tumor necrosis factor-α (TNF-α).

Question 24

IL-1 is associated with fever. IL-2 activates T cells. IL-6 is an acute-phase reactant seen in states of inflammation. Interferon-γ promotes vascular permeability and T-cell activation. TNF-α activates endothelium and causes white blood cell recruitment. As a result of release of these cytokines, patients develop septic shock (ie, fever, hypotension, tachycardia, warm extremities). In the case of TSS, patients often exhibit a diffuse red rash on all body surfaces with desquamation (as shown the image). Characteristically, the desquamation occurs on palmar and plantar surfaces.

Although TSS was once strongly associated with the use of highly absorbent tampons, the withdrawal of these tampons from the market has caused ?

Answer 24

menstruation-related cases to decrease over time, so that they now represent less than half of all cases.

The other answer choices list cytokines that are not involved in the inflammatory response and/or fail to include all the relevant cytokines. IL-3 is important for bone marrow stimulation. IL-10 is secreted by regulatory T cells to attenuate the inflammatory response. IL-12 activates natural killer cells and promotes differentiation of Th1 cells.

Question 25

A history of retained foreign bodies in the nose or vagina in the setting of shock (ie, fever, hypotension, tachycardia, altered mental status, warm extremities, etc) should raise suspicion for staphylococcal toxic shock syndrome (TSS); the typical desquamating rash should raise it to the top of the differential diagnosis list. Commonly, TSS is mediated by ?

Answer 25

Staphylococcus aureus through nonspecific superactivation of T cells by TSS toxin. The elevated release of IL-1, IL-2, IL-6, IFN-γ, and TNF-α mediates many of the clinical manifestations of shock.

Question 26

This patient presents with fever, malaise, chills, new-onset cardiac murmur, and skin lesions on both arms. These are classic symptoms of acute bacterial endocarditis (ABE). Endocarditis is often characterized by constitutional symptoms and cardiac murmur. Other symptoms include?

Answer 26

Janeway lesions (macular or nodular lesions on the palms and soles), Osler nodes (red raised lesions on the hands and feet), and Roth spots (retinal hemorrhages with white or pale centers).
Acute and subacute endocarditis can be differentiated based on history. The acute form will have the most severe and sudden onset, as seen in this patient. ABE is seen most often in patients who use intravenous drugs or have indwelling catheters. S. aureus is the most common bacterial pathogen isolated from these patients because it is part of the skin flora and enters the blood at injection sites.

Question 27

This patient has a history of intravenous drug use, as indicated by the multiple punctate lesions on his arms. Together with the auscultation of a murmur consistent with tricuspid regurgitation, these findings point to a right-sided ABE infection. Right-sided ABE is often characterized by septic emboli to the lungs, leading to bilateral infiltrates. This patient is manifesting signs of bilateral infiltrates with hypoxia, decreased breath sounds, and dullness to percussion. It is important to note that many of the classic signs of?

Answer 27

endocarditis, such as Janeway lesions, are usually a complication of left-sided endocarditis.

Viridans streptococci would cause mitral valve infective subacute endocarditis, whereas Streptococcus bovis most commonly causes aortic valve infective endocarditis. Enterococcus faecalis is not a likely cause, since there is no gastrointestinal or urologic damage. This patient’s onset of symptoms is too acute to be related to Haemophilus aphrophilus. Infection with Pseudomonas aeruginosa in patients who use intravenous drugs causes osteomyelitis more frequently than endocarditis.

Question 28

A new-onset murmur in patients who use intravenous drugs is most likely a sign of endocarditis caused by ??

Answer 28

S. aureus.

Question 29

A microbiology laboratory is investigating the possibility of using enzymes as a novel class of antibacterial. These enzymes cannot traverse the double-layer lipid membrane. Instead, they bind directly to the surface peptidoglycans and hydrolyze bonds within them.

Which of the following organisms would be a logical target for this type of drug?

Answer 29

Knowledge of these enzymes is not required to answer the question. It is simply testing for a basic understanding of bacterial cell structure. The question states that the enzymes are able to bind surface peptidoglycans directly from the extracellular space. Peptidoglycans make up bacterial cell walls. In other words, the question is merely asking: “In what type of bacteria is the cell wall the outermost structure?” The answer is the gram-positive bacteria, which have a thick peptidoglycan cell wall. Of the answer choices, the only gram-positive bacterium is Staphylococcus aureus.

Question 30

Brucella abortis is a gram-negative bacterium.

Chlamydia trachoma is an intracellular pathogen, and would be inaccessible to enzymes in the extracellular space.

Escherichia coli is a gram-negative organism. Thus it has an outer membrane surrounding a thin peptidoglycan layer. The cell wall would not be directly accessible to an enzyme in the extracellular space.

Ureaplasma urealyticum lacks a?

Answer 30

cell wall entirely.

For a hypothetical antibiotic that hydrolyzes surface peptidoglycans, gram-positive bacteria would be most susceptible since their outermost structure is a thick peptidoglycan cell wall. Intracellular bacteria, bacteria without cell walls, or gram-negative bacteria, whose thin peptidoglycan cell wall is enclosed by an outer lipopolysaccharide membrane, would be less susceptible to this mechanism of action.