Micro USMLE 8-27(8)

Question 1

This elderly man presents with productive cough, fever, and bronchial breath sounds, as well as a high fever and diarrhea. His laboratory values reveal thrombocytopenia, elevated creatinine and blood urea nitrogen (BUN) levels, hyponatremia, and hypophosphatemia. When combined with his other signs and symptoms, these findings suggest that the patient mostly likely has Legionella pneumonia. Legionella organisms can be identified by means of?

Answer 1

urine antigens and growth on buffered charcoal yeast extract agar.

Question 2

The single most important test for diagnosis of Legionnaires’ disease is isolation of the organisms using a?

Answer 2

buffered charcoal yeast extract agar, which contains L-cysteine, iron, vancomycin, and dyes to prevent the overgrowth of competing organisms and to stain the organism of interest. Recently, urine antigen testing has been developed for Legionella. This technique is much faster than culturing Legionella and is being used more frequently for presumptive diagnosis, with the culture serving as a confirmatory test.

Question 3

Myobacterium tuberculosis is cultured on Löwenstein-Jensen agar; however, individuals infected with this pathogen do not present with diarrhea or hyponatremia.

Nutrient agar is nonspecific and is the all-purpose agar for bacterial culture. Staining with India ink is used to detect Cryptococcus neoforman, but this fungus causes opportunistic infections, classically meningoencephalitis, in an immunocompromised host. (C. neoformans can also cause pneumonia.)

Ziehl-Neelsen stain is used to identify?

Answer 3

Mycobacterium and other acid-fast bacteria; however, infections with these organisms do not typically present with diarrhea and hyponatremia.

Question 4

When Legionella pneumophila infection is suspected, the appropriate culture medium is?

Answer 4

buffered charcoal yeast extract agar with iron and cysteine in combination with urinary antigen testing. Typically, Gram staining reveals large neutrophils but no organisms.

Question 5

The patients in this community developed sudden-onset hyperpyrexia, cough, and malaise. On examination they were found to have rales and bronchial breath sounds. These symptoms are suggestive of pneumonia. A culture of a patient’s sputum reveals the causative pathogen: ?

Answer 5

Legionella pneumophila bacteria. L. pneumophila is an aerobic, gram-negative rod that causes Legionnaires’ disease, a condition in which patients develop acute, severe pneumonia, diarrhea, hyponatremia, and high fever. Legionnaires’ disease is an often overlooked cause of atypical community-acquired pneumonia and is identified as the cause in only 3% of cases. The organism is present only in water sources (eg, air-conditioning systems, whirlpools, mist sprayers) and is not spread by person-to-person contact. Typically, more severe illness is seen in patients who are =50 years of age and those who smoke. Silver stain can be used to visualize L. pneumophilia, which stain black to brown against a yellow background. Gram stain shows neutrophils and very few organisms, as L. pneumophila is facultatively intracellular.

Question 6

Infection with Bordetella pertussis does not cause pneumonia and presents with paroxysms of coughing. Haemophilus influenzae is associated with epiglottitis or meningitis. Mycobacterium tuberculosis reactivation (or secondary tuberculosis) manifests as a chronic, low-grade fever with night sweats, malaise, and weight loss. Streptococcus pneumoniae is an important cause of ?

Answer 6

community-acquired pneumonia and may present with symptoms that are similar to those seen in these patients. However, the sputum culture for S. pneumoniae infection would reveal significant growth of gram-positive diplococci.

Question 7

Legionella pneumophila infection causes Legionnaires’ disease, a form of atypical community-acquired pneumonia. This pathogen should be suspected in patients with?

Answer 7

severe pneumonia with acute hyperpyrexia, diarrhea, and hyponatremia.The organism L. pneumophila can be visualized on silver stain, but Gram stain will show neutrophils and poorly staining organisms.

Question 8

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 8

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 9

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 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?

Answer 9

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 10

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 10

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 11

Staphylococcus aureus is a gram-positive, catalase-positive, coagulase-positive organism that is a common cause of septic arthritis, and it can be identified by?

Answer 11

the appearance of cocci in clusters on Gram staining.

Question 12

The enzymes that are being investigated can bind surface peptidoglycans directly from the extracellular space; they are unable to traverse double-layer lipid membranes. Peptidoglycan is a component of both gram-positive and gram-negative cell walls but is distinctly located in each. Gram-positive bacteria have a thick peptidoglycan cell wall as the outermost structure. Gram-negative bacteria also have a peptidoglycan layer, but it is surrounded by an outer lipid bilayer (see image below). Thus these enzymes will only affect gram-positive bacteria. Of the answer choices, the only gram-positive bacterium is ?

Answer 12

Staphylococcus aureus.

A gram-positive bacterium, such as Staphylococcus aureus, would be most susceptible to a hypothetical antibiotic that hydrolyzes surface peptidoglycans because its outermost structure is a thick peptidoglycan cell wall. An intracellular bacterium, a bacterium without a cell wall, or a gram-negative bacterium with a thin peptidoglycan cell wall enclosed by an outer membrane would not be susceptible to the enzymes being investigated.

Question 13

Eschericia coli, Brucella abortis, and Chlamydia trachomatis are all gram-negative bacteria and therefore have an outer lipid bilayer, which cannot be traversed by these enzymes. C. trachomatis is also an obligate intracellular bacterium, providing an additional barrier to the enzymes. Ureaplasma urealyticum lacks ?

Answer 13

a cell wall entirely and therefore would be unaffected by peptidoglycan-destabilizing enzymes.

Question 14

Neisseria meningitidis is a gram-negative diplococcus that is a common cause of bacterial meningitis. It is often associated with?

Answer 14

a petechial rash in children and young adults. In severe cases, it can cause Waterhouse-Friderichsen syndrome, a condition characterized by sepsis and adrenal insufficiency due to adrenal hemorrhage.

Question 15

The boy is brought in to see his physician after multiple vomiting episodes and a fever. His symptoms—including fever, tender neck, and maculopapular rash with petechiae on his trunk—and laboratory findings are consistent with meningitis and septicemia caused by?

Answer 15

Neisseria meningitidis. This gram-negative diplococcus is the second most common cause of meningitis in children aged 6 months to 6 years and is the leading cause of meningitis in older adolescents and young adults. The bacterium grows best on Thayer-Martin VCN medium, which contains antibiotics that kill competing bacteria and fungi. However, if it is obtained from sterile sources such as cerebral spinal fluid (CSF), it can be isolated on nonselective media.

Question 16

The CSF sample would most likely show a white blood cell count > 1000/mm3 with mostly neutrophils, glucose < 40 mg/dL, and protein 100–500 mg/dL, in keeping with the bacterial origin. Once in the bloodstream, the bacteria can cause a characteristic petechial rash (like that shown in the image) caused by endotoxin, which predisposes an individual to?

Answer 16

Waterhouse-Friderichsen syndrome (sepsis with adrenal insufficiency and potential adrenal hemorrhage). Adrenal insufficiency presents with anorexia, nausea, vomiting, abdominal pain, weakness, fatigue, fever, and confusion; it can even cause coma.

Question 17

Ascending peripheral nerve paralysis would be associated with Guillain-Barré syndrome, which may occur after infection with Campylobacter jejuni.
Myocardial infarction would be associated with Kawasaki disease, a medium-sized arterial vasculitis, which can occur in children.
Sudden onset of fever, lethargy, and a new heart murmur would be associated with?

Answer 17

acute bacterial endocarditis.
Right-sided upper and lower facial droop would be associated with Bell palsy, a possible sequela of herpes simplex virus activation or Lyme disease.

Question 18

This girl’s symptoms began 4 days ago with a fever, headache, nausea, and a rash on her cheeks. She now presents to the emergency department complaining of a rash on her trunk and legs. Her temperature and white blood cell count are both elevated. These findings are highly suggestive of fifth disease.
Parvovirus B19 is a ?

Answer 18

single-stranded negative DNA virus that is the causative organism of fifth disease (also called erythema infectiosum), which typically affects children 5–10 years old. This condition is characterized by an initial prodromal phase that includes findings such as a low-grade fever, headache, and gastrointestinal symptoms. The virus causes an erythematous rash that looks like both cheeks have just been slapped (like shown in this image). In addition, a pruritic maculopapular rash starts on the arms and spreads to the trunk and legs. Parvovirus also causes aplastic anemia in patients with sickle cell anemia and hereditary spherocytosis.

Question 19

Single-stranded positive RNA viruses (such as coxsackie virus), single-stranded enveloped negative RNA viruses (such as paramyxovirus), and single-stranded enveloped positive RNA viruses (such as the rubella virus) do not present with a slapped cheek rash as presented in this patient’s symptoms. Catalase-negative, coagulase-negative diplococci describe?

Answer 19

streptococcus bacteria species which produce diseases such as pharyngitis, cellulitis, and scarlet fever.

Parvovirus B19 is the causative organism of fifth disease (erythema infectiosum), which is characterized by an erythematous rash on both cheeks.

Question 20

In 1918–1919, a virus was responsible for the deaths of 20-40 million people worldwide. This is more people than were killed during World War I, and more than were killed during 4 years of the bubonic plague.

Which of following characteristics of the virus most likely caused this abnormally large incidence of infections?

Answer 20

It is a segmented virus.

The event described in the vignette was caused by the influenza virus, which is characterized by headache, fever, malaise, and upper respiratory symptoms. The number of people this affected is typical of a pandemic. Compared with an epidemic, a pandemic refers to the spread of an infectious disease that affects a much larger population and across continents.
Influenza pandemics are the result of genetic/antigenic shift, a phenomenon that occurs when viruses with segmented genomes (eg, influenza virus) exchange segments, resulting in reassortment or transfer of genetic information. In the case of influenza, this genetic information is hemagglutinin and neuraminidase antigens. The result is a new virus with a combination of surface antigens that has never been exposed to a human immune system anywhere on the planet. Thus, the entire human population would be susceptible, leading to a pandemic. Other commonly tested segmented viruses include Bunyaviridae, Arenoviridae, and Reoviridae.

Question 21

Non-segmented viruses do not cause pandemics, since they are unable to undergo reassortment, which is one contributing factor to pandemics. In contrast to antigenic shift, antigenic drift is the gradual accumulation of mutations of the epitope of the virus that changes the recognizance of the virus by our immune system. Antigenic drift is responsible for ?

Answer 21

seasonal fluctuations in influenza virulence and the reason that the efficacy of the annual flu vaccine varies depending on its match to the influenza strain expected to predominant in a given year. Similarly, the lack of RNA proofreading of some viruses leads to minor changes in virulence as mutations accumulate.

Question 22

This patient presents with fever, dyspnea on exertion, and unintentional weight loss, which are suggestive of pneumonia. He also has a history of unprotected sex with multiple partners, and presents with oral thrush and inguinal lymphadenopathy, which are suggestive of HIV. Taken together, the patient’s presentation and history raise suspicion of an opportunistic infection, such as Pneumocystis jirovecii (PCP) pneumonia. PCP pneumonia is an AIDS-defining illness and is typically seen in patients with CD4 counts <200 cells/mm. P. jirovecii is an ascomycetous fungi that also causes pneumonia in hematopoietic-cell and solid-organ transplant recipients, cancer patients, and patients receiving glucocorticoids, chemotherapeutic agents, and other immunosuppressive medications. The standard treatment for this illness is ?

Answer 22

trimethoprim-sulfamethoxazole (TMP-SMX), a combination antibiotic that synergistically inhibits folate synthesis by inhibiting dihydrofolate reductase and dihydropteroate synthetase. If the patient has a sulfa-drug allergy, the treatment of choice would be pentamidine.

Question 23

Penicillins works by inhibiting the transpeptidase crosslinking of the peptidoglycan cell wall. They are most effective against gram-positive organisms such as Streptococcus pneumoniae, Streptococcus pyogenes, and Actinomyces. Tetracyclines act by binding to the 30S-ribosomal subunit and preventing attachment of the aminoacyl-transfer RNA; they are used to treat infections caused by Mycoplasma pneumoniae. Macrolides are a class of antibiotics that act by binding to?

Answer 23

the 23S-ribosomal RNA of the 50S-ribosomal subunit, thereby inhibiting protein synthesis by blocking translocation. Macrolides are first-line medications for the treatment of pneumonia caused by S. pneumoniae, the most common cause of community-acquired lobar pneumonia, and M. pneumoniae, the most common cause of atypical pneumonia. Fluoroquinolones work by inhibiting topoisomerase II (DNA gyrase) and topoisomerase IV. This is the drug of choice to treat urinary tract infections caused by Escherichia coli, which is a gram-negative bacteria.

Question 24

Consider Pneumocystis jirovecii pneumonia in the immunocompromised patient presenting with progressively worsening cough, fevers, malaise, and shortness of breath. The treatment of choice is ?

Answer 24

trimethoprim-sulfamethoxazole.

Question 25

This patient’s presentation is highly suggestive of bacterial meningitis. In the neonate, clinical symptoms are nonspecific (fever, irritability, lethargy, poor feeding) relative to those in older children or adults (nuchal rigidity). The most common cause of neonatal meningitis is?

Answer 25

group B Streptococcus (GBS), specifically Streptococcus agalactiae, a β-hemolytic, gram-positive coccus found in chains (see image). In contrast with its group A counterparts, S. agalactiae is bacitracin resistant.
Early-onset GBS infection usually manifests in the first 24 hours of life. It occurs through vertical transmission from the birth canal during delivery, as 40% of women are asymptomatic carriers of this bacterium in the gastrointestinal tract and vagina. Prenatal screening for GBS colonization has greatly decreased the number of early-onset GBS infections. Late-onset GBS infection occurs in the first 4–5 weeks of life and is independent of prenatal care. It occurs as a result of community acquired GBS infection and is occasionally preceded by an upper respiratory infection.

Question 26

A gram-negative bacillus that ferments lactose refers to Escherichia coli, and is the second most common cause of neonatal meningitis after GBS.
A gram-negative coccobacillus that grows on chocolate agar with factors V and X refers to Haemophilus influenzae type B (Hib). This was once a major cause of serious bacterial infections, including meningitis, sepsis, and epiglottitis. However, with the advent of the Hib vaccine, rates have decreased significantly.

A gram-positive bacillus that is facultative intracellular refers to?

Answer 26

Listeria monocytogenes, which is the third most common cause of neonatal meningitis (2%).
A gram-positive coccus that is α-hemolytic, optochin-sensitive, and bile-soluble refers to Streptococcus pneumoniae, which is the leading cause of meningitis starting at 6 months of age.

Question 27

Streptococcus pneumoniae is a gram-positive coccus that is lancet-shaped, found in chains, and encapsulated (positive quellung test). It is not a significant cause of meningitis in neonates. However, in children ages 6 months to 6 years, it is the leading cause of ?

Answer 27

meningitis.

Question 28

This patient has systemic symptoms of fever, night sweats, and weight loss, which are likely associated with an infection caused by ?

Answer 28

her severe immunosuppression with a CD4+ count of less than 50 cells/mm³. Disseminated Mycobacterium avium–intracellulare can manifest as a severe systemic illness in immunocompromised individuals when the CD4+ count is less than 50 cells/mm³. Prophylaxis with azithromycin is at 50 cells/mm³.

Question 29

Herpes simplex virus infection can occur when the CD4+ count is less than 400/mm³ or earlier. Infection typically presents with oral or genital ulcers.
Herpes zoster infection can manifest when the CD4+ count is less than 400/mm³ or earlier. It is characterized by a painful collection of vesicles in a dermatomal pattern.
Oral thrush can occur when the CD4+ count is less than 500/mm³. It manifests with white patches and plaques on the oral mucosa.
Pneumocystis pneumonia is a common opportunistic infection when the CD4+ is less than 200 cells/mm³. It commonly manifests with?

Answer 29

fever, malaise, dyspnea on exertion, hypoxia, and a nonproductive cough.
A Toxoplasma brain abscess may occur when a patient’s CD4+ count is less than 100 cells/mm³. It most commonly manifests with focal neurologic deficits, and a CT scan will show multiple ring-enhancing lesions.

Question 30

Immunosuppression from HIV infection leads to susceptibility to various opportunistic infections, depending on a patient’s CD4+ count. CD4+ counts less than 50/mm³ predispose HIV-positive patients to Mycobacterium avium–intracellulare (MAC) infections, which can be identified by symptoms resembling those of ?

Answer 30

tuberculosis—fever, night sweats, and weight loss. Prophylactic azithromycin is administered to patients to prevent MAC infections.