Micro USMLE 8-20 (1)

Question 1

The patient’s symptoms of pleuritic chest pain, shortness of breath, productive cough with rust-colored sputum, and fever are suggestive of?

Answer 1

pneumonia. Furthermore, the results of the sputum culture confirm that the most likely cause is Streptococcus pneumoniae; sputum culture stains positive for Gram stain and S. pneumoniae typically have a lancet shaped diplococci, as seen the in the image above.

Question 2

S. pneumoniae is the most frequent cause of ?

Answer 2

community-acquired and lobar pneumonias. The image usually shows right lower lobe consolidation and lobar effusion characteristic of lobar pneumonia.

Question 3

The other answer choices are associated with ayptical pneumonia, which is associated with inflammation of the interstitium. Atypical pneumonia typically does not cause consolidation of the lung. Lobar, rather than patchy, consolidation is seen in pneumococcal pneumonia. Atypical pneumonia is typically caused by ?

Answer 3

viruses and bacteria such as Mycoplasma pneumoniae and Chlamydia pneumoniae, which do not appear on Gram stain.

Question 4

The most common organism causing lobar pneumonia is Streptococcus pneumoniae. Lobar pneumonia is characterized by?

Answer 4

shortness of breath, a cough productive of rust-colored sputum, Atypical pneumonia, by contrast, is more often caused by Mycoplasma, Chlamydia, or viruses.

Question 5

Sensitivity to optochin is characteristic of Streptococcus pneumoniae, the most common cause of community-acquired pneumonia. Suspect pneumococcal pneumonia in a patient with lobar findings on physical exam (dullness to percussion, bronchial breath sounds, and egophony in a focal area), as well as lobar consolidation on chest radiograph. This patient lives in the community and has no comorbid conditions or recent hospitalizations that would predispose her to?

Answer 5

hospital-acquired organisms or pulmonary infections contracted by travelers; this makes community-acquired pathogens most likely.

Question 6

Bacitracin sensitivity is indicative of Streptococcus pyogenes. S. pyogenes is a common cause of pharyngitis, cellulitis, and impetigo but does not cause lobar pneumonia.

Coagulase positivity is indicative of Staphylococcus such as S. aureus infection, which typically manifests as a patchy bronchopneumonia that can evolve into a lobar pneumonia if the infection goes untreated. It is often seen in elderly or younger patients after a viral infection. S. aureus also can cause empyema, necrotizing pneumonia, or pulmonary abscesses.

The ability to ferment lactose is characteristic of ?

Answer 6

Klebsiella, a common cause of pneumonia in alcoholic and diabetic patients. Other features of Klebsiella infection include bloody “currant jelly” sputum and cavitations on X-ray of the chest.

The presence of cold agglutinins in serum is indicative of Mycoplasma pneumoniae, the most common cause of atypical pneumonia. Atypical, or “walking,” pneumonia has an insidious onset and is most likely to manifest in younger adults. X-ray of the chest often reveals patchy infiltrates rather than lobar consolidation. Patients also can have extrapulmonary manifestations, the most serious of which include neurologic symptoms (aseptic meningitis, peripheral neuropathy, and ataxia).

Question 7

This patient with hemoptysis, chest pain, fever, malaise and a lobar consolidation on chest x-ray has most likely developed ?

Answer 7

community acquired pneumonia (CAP). Additional symptoms of CAP include a productive cough, chills, and headache.

Question 8

Patients who have sickle cell anemia usually develop autosplenectomy at an early age due to chronic blood flow occlusion from sickle shaped red blood cells, and are therefore at increased risk of developing infection from encapsulated organisms, such as Streptococcus pneumoniae. Vaccination against common encapsulated bacterial organisms is recommended for all patients who have undergone splenectomy of any kind. As this patient has a poor history of follow up for her sickle cell anemia, it is likely that she has not been vaccinated. For patients with extra risk factors or who require hospitalization, first-line treatment for a community-acquired pneumonia is?

Answer 8

either a macrolide, such as azithromycin, plus a third-generation cephalosporin or a respiratory fluoroquinolone (levofloxacin or moxifloxacin). First-line treatments for outpatient care of CAP are tetracyclines or a macrolide.

Question 9

Ciprofloxacin is early generation fluoroquinolone that is used mainly to treat gram-negative rods and is not used for respiratory infections. Gentamicin is an aminoglycoside used to treat gram-negative microbial infections. Piperacillin-tazobactam is used to treat ?

Answer 9

drug resistant infections. Trimethoprim-sulfamethoxazole is typically used to treat urinary tract infection due to Escherichia coli or Enterobacter species.

Question 10

Streptococcus pneumoniae, a gram-positive diplococcus, is the most common cause of community acquired pneumonia (CAP) in adults. First-line treatment for a community-acquired pneumonia in a patient with extra risk factors, or who requires hospitalization, is either a ?

Answer 10

macrolide, such as azithromycin, plus a third-generation cephalosporin or a respiratory fluoroquinolone, such as levofloxacin or moxifloxacin. First-line treatment for CAP in an otherwise healthy patient is a macrolide or tetracycline.

Question 11

This young patient’s clinical presentation and cerebrospinal fluid (CSF) analysis are consistent with a diagnosis of bacterial meningitis. The most common cause of bacterial meningitis in children between 3 months and 10 years of age is Streptococcus pneumoniae, which shows up as?

Answer 11

gram-positive diplococci on Gram stain (shown here). In adolescents and young adults, meningitis caused by Neisseria meningitidis is more common, but Streptococcus pneumoniae again becomes the most common agent in older adults.

Question 12

Neisseria meningitidis would be indicated by gram-negative diplococci. It is more common in adolescents and young adults, specifically between the ages of 10 and 24 years of age.
Haemophilus influenzae and Escherichia coli are gram-negative rods. H. influenzae is a less common cause of meningitis in children of this age group due to the Hib vaccine, and E. coli is a common culprit of meningitis in newborns.
Listeria species are?

Answer 12

gram-positive rods that are much more commonly seen in newborns (age 0–6 months) and the elderly.
Viral meningitis would result in a negative Gram stain.

Question 13

A 45-year-old man presents to his primary care physician with the recent onset of productive cough, dyspnea, and fever. A sample of the patient’s sputum is smeared onto a blood agar plate, which grows several colonies that turn the surrounding medium green. An optochin disc placed on one colony inhibits all growth surrounding the disc.

Which virulence factor contributes to this organism’s ability to colonize the lung?

Answer 13

The organism identified is Streptococcus pneumoniae. S. pneumoniae can be distinguished from other microorganisms by virtue of its ability to produce α-hemolysis (turning blood agar green) and its sensitivity to optochin. Another important feature of S. pneumoniae is its ability to produce IgA protease enzymes that cleave secretory IgA antibodies.

Question 14

Secretory IgA normally provides the major defense for mucosal surfaces against bacterial colonization. The compromise of this defense system strongly contributes to the virulence of S. pneumoniae. Other bacteria that secrete IgA proteases include Neisseria meningitidis, N. gonorrhoeae, and Haemophilus influenzae. Another key virulence factor of S. pneumoniae, and the MOST important for?

Answer 14

colonizing the lungs, is the presence of a polysaccharide capsule, which enables the organism to resist phagocytosis.

Question 15

Rhinovirus uses ICAM-1 (intracellular adhesion molecule-1), a protein expressed on respiratory epithelium, as a mode of entry to initiate infection. Rhinovirus is a common cause of upper respiratory infections.

Lipopolysaccharide (LPS), also known as endotoxins, are composed of a lipid and polysacharide, found on the outer membrane of Gram-negative bacteria. It induces strong immune response in the host.

Pili can contribute to bacterial cell adhesion and virulence but are found predominantly on?

Answer 15

gram-negative bacteria.

Protein A binds Fc-IgG and inhibits complement fixation and phagocytosis. However, it is a virulence factor associated with Staphylococcus aureus, not Streptococcus pneumoniae.

Question 16

The patient arrives with otitis externa, which often occurs after swimming. Otitis externa typically manifests with ear pain, pruritus, discharge, and pain on manipulation of the pinna. Periauricular cellulitis and fever are signs of more serious infection. Progression of this disease can lead to?

Answer 16

osteomyelitis of the bones of the skull and cranial nerve damage. As opposed to otitis media, with otitis externa there is typically no evidence of middle ear fluid. Pseudomonas aeruginosa is the most common cause of otitis externa (39% of cases).

Question 17

Haemophilus influenzae, prior to the HiB vaccine, commonly caused otitis media and meningitis, but not otitis externa.
Moraxella catarrhalis is another common cause of otitis media and pneumonia, especially in COPD patients.

Streptococcus pyogenes, or group A streptococci, causes a?

Answer 17

litany of infections, including cellulitis, pharyngitis, scarlet fever, and impetigo. Although it is sometimes implicated in otitis media, it rarely causes otitis externa.
Streptococcus pneumoniae is the most common cause of otitis media but not otitis externa.

Question 18

Otitis externa is an acute infection of the ear canal, typically caused by bacteria, with Pseudomonas aeruginosa being the most frequent causative agent, especially after swimming. Otitis media is most commonly caused by?

Answer 18

Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis.

Question 19

This man presenting with high fever, shaking chills, productive cough, lobar consolidation on chest X-ray, and a sputum culture that reveals an oxidase-negative pathogen is most likely to have community-acquired pneumonia (CAP). Although Streptococcus pneumoniae is the most common cause of CAP, the Gram stain reveals gram-negative rods.
In cases of pneumonia in patients who are malnourished or debilitated, or have a history of alcoholism, the most likely?

Answer 19

gram-negative infection is Klebsiella pneumoniae. Gram-negative rods often respond well to ceftriaxone, a third-generation cephalosporin; however, susceptibility testing will determine the appropriate antibiotic in different hospital settings.

Question 20

Some experts suggest using combined therapy for CAP caused by K. pneumoniae, if this gram-negative pathogen does not produce extended-spectrum beta lactamases, by adding an aminoglycoside, but this treatment remains under debate. A helpful mnemonic for the characteristics of pneumonia caused by Klebsiella infections is?

Answer 20

the 5 A’s: Aspiration pneumonia, Abscess in lung and liver, Alcoholism, currAnt-jelly sputum, and diAbetes.

Amphotericin B is used for fungal infections, not bacterial pneumonia. While isoniazid and vancomycin are antibiotic treatments, as is clavulanic acid when combined with penicillin-group antibiotics, they are not effective in this gram-negative infection.

Question 21

Community-acquired pneumonia caused by Klebsiella pneumoniae, a gram-negative rod, often affects patients who are malnourished and debilitated, including individuals with a history of?

Answer 21

chronic alcoholism. Ceftriaxone, with or without an aminoglycoside, would be an appropriate treatment.

Question 22

The patient presents with an abscess that is draining pus. Microscopic examination shows yellowish granules. The characteristic “sulfur granules,” which can be seen under the microscope and may drain through sinus tracts in skin, are indicative of an infection caused by Actinomyces israelii. A stained sample from the abscess shows branching, gram-positive rods, which further confirm?

Answer 22

Actinomyces israelii as the culprit.

Actinomyces israelii is a microaerophilic organism that can cause oral and facial abscesses. It is part of the normal flora of the mouth and gastrointestinal tract of humans, and infection is initiated usually by trauma. Treatment for this infection is penicillin.

Question 23

As the bacteria is gram positive, metronidazole provides no benefit. Nystatin is only for fungal infections, and oxacillin is mainly used for?

Answer 23

methicillin-sensitive S. aureus. While Nocardia spp. are weakly acid-fast and can be confused with Actinomyces, they are susceptible to sulfonamides, not penicillin.

Question 24

The classic presentation of an infection with the gram positive Actinomyces bacteria are?

Answer 24

mandibular pain after a dental procedure and draining pus that reveals sulfur (“yellowish”) granules when viewed through microscope. Treatment for Acintomyces includes penicillin.

Question 25

This patient’s fever in the setting of peritoneal dialysis with erythema around the catheter site suggests an infection likely involving bacteria normally present on the skin. Staphylococcus epidermidis (S. epidermidis) and other coagulase-negative staphylococci are part of the normal human skin flora and can cause systemic infection through entry sites into the body such as Foley urinary catheters, intravenous lines, prosthetic devices, and peritoneal dialysis catheters. S. epidermidis can migrate along the tubing with the help of biofilm creation. Fevers, chills, diffuse abdominal tenderness, and rebound tenderness all suggest a diagnosis of ?

Answer 25

peritonitis, infection / inflammation of the peritoneal cavity, which can be caused by many bacteria, including S. epidermidis. This pathogen is also a common cause of endocarditis in patients with prosthetic valves and of intravenous line-associated infections. S. epidermidis is a gram-positive, coagulase-negative coccus.

Question 26

The gram negative bacteria and acid fast bacilli described in the other answer choices rarely cause catheter-related infections as they are not typical skin microbes and are less common causes of device-associated infections than the coagulase-negative staphylococci. Bacteroides fragilis, a gram-negative, obligate anaerobic bacilli, is a normal member of the gastrointestinal flora and can cause?

Answer 26

peritoneal infections when the integrity of the bowel is compromised; it is rarely associated with catheter-associated peritonitis. Proteus mirabilis, a gram-negative, non-lactose fermenting, urease-positive bacilli, can cause UTIs, as well as bacterial peritonitis.

Question 27

Klebsiella pneumoniae, a gram-negative, lactose-fermenting, urease-positive bacilli, is implicated in aspiration pneumonia and many other infections, including peritoneal infections, when the integrity of the bowel is compromised.
Mycobacterium tuberculosis is a gram-positive, acid-fast bacillus that is associated with ?

Answer 27

pulmonary infection, though bovine tuberculosis associated with infected milk can present as a subacute peritonitis.

Question 28

Staphylococcus epidermidis (and other coagulase-negative staphylococci) is a part of normal skin flora and can cause infections, including peritonitis, in the setting of indwelling prosthetic devices or catheters. It is the most common cause of ?

Answer 28

dialysis-associated peritonitis, with Staphylococcus aureus being the second most commonly implicated pathogen.

Question 29

This patient presents with a history of intravenous (IV) drug use and symptoms of fever, chills, and pleuritic chest pain with bacteremia. This patient most likely has infective endocarditis of the tricuspid valve. This valve is commonly affected in those with a history of IV drug use. The vegetations that exist on the tricuspid valve can detach, embolize, and lodge themselves in the pulmonary circulation, resulting in ?

Answer 29

a pulmonary embolism that prevents blood flow to the left side of the heart.
This process is similar to a deep venous thrombosis (DVT), in which a clot embolizes and lodges itself in the pulmonary circulation, causing a pulmonary infarct. As a result, blood flow to the left side of the heart is blocked. Of note, the image provided with the question shows splinter hemorrhages, which are microemboli caused by a similar mechanism, sent to the periphery. Blood cultures for patients with tricuspid valve endocarditis are usually positive, and the most commonly implicated organism is Staphylococcus aureus.

Question 30

The murmur of tricuspid valve endocarditis is best auscultated over?

Answer 30

the left fourth intercostal interface, just lateral to the sternal border.
On the chest x-ray (see left panel of image), we can see multiple wedge-shaped infarcts that are consistent with pulmonary embolism, and on the CT scan (see right panel of image), we can see the feeding vessel sign, indicating the lesion has a hematogenous origin or a disease process near the small pulmonary vessels.

Cardiac tamponade, Dressler syndrome, left-sided heart failure, and stable angina are not caused by an embolic event.