Micro USMLE 8-27(9)

Question 1

This patient presents with symptoms of knee pain, conjunctivitis, and urethral discharge. This triad is a classic presentation of reactive arthritis (formerly called Reiter syndrome). Reactive arthritis is an acute spondyloarthropathy that typically manifests 1–3 weeks after a sexually transmitted or enteric infection with ?

Answer 1

Chlamydia trachomatis, Neisseria gonorrhoeae, Salmonella, Shigella, Yersinia, Campylobacter, or Ureaplasma urealyticum. Affected patients are usually male, with the human leukocyte antigen-B27 phenotype. Culture of synovial fluid drawn from an affected joint is usually negative, suggesting an autoimmune-mediated reaction.

Question 2

Signs of reactive arthritis include arthritis (often involving large joints within the lower extremities, on one side), enthesopathy, mucocutaneous lesions, conjunctivitis (see image), and nonpurulent genital discharge (as in this patient). A useful mnemonic for the classic triad of knee pain, conjunctivitis, and urethral discharge is ?

Answer 2

“Can’t pee, can’t see, can’t climb a tree.” Nonsteroidal anti-inflammatory drugs (NSAIDs) are the first-line treatment, though sulfasalazine or immunosuppressants are sometimes used.

Question 3

IgM antibodies against IgG are associated with rheumatoid arthritis.
Anti-Ro and anti-La antibodies are found in ?

Answer 3

Sjögren syndrome.

Double-stranded DNA antibodies are present in lupus.

Question 4

Reactive arthritis is a post-infectious process characterized by conjunctivitis, urethritis, and arthritis. Joint aspirates of synovial fluid are usually uninfected, differentiating this illness from?

Answer 4

septic arthritis.

Question 5

his patient presents with pain and decreased range of motion of her left knee after a knee replacement surgery. She also shows signs of infection with her fever and erythematous knee. These findings combined with the recent surgical trauma and new prosthetic joint are suggestive of septic arthritis. Overall, Staphylococcus aureus is the most common offending organism in cases of septic arthritis in adults and children.
S. aureus is?

Answer 5

a gram-positive, catalase-positive cocci. However, Staphylococcus epidermidis is also a gram-positive, catalase-positive cocci and is known for causing nosocomial infections of implanted foreign bodies such as catheters, joint prosthetics, and prosthetic heart valves. The correct answer does not further differentiate which bacterium is the offending one, but coagulase testing would reveal the difference. S. aureus is coagulase-positive whereas S. epidermidis is coagulase-negative. In either case, we can establish that both bacteria are gram-positive, catalase-positive cocci.

Question 6

A gram-negative lactose-fermenting bacilli describes E. coli which is usually found in UTIs. Gram negative non-lactose fermenting bacilli describes Salmonella which would present with bloody diarrhea and GI symptoms. Additionally, gram-negative glucose fermenting cocci would be descriptive of ?

Answer 6

Neisseria which is found in cases of meningitis and sexually transmitted diseases. Gram-positive catalase negative cocci describes Streptococcus species, which is often found in diseases such as pharyngitis and scarlet fever.

Question 7

Staphylococcus aureus and Staphylococcus epidermidis are gram-positive, catalase-positive cocci that are likely to cause?

Answer 7

septic arthritis after joint replacement surgery.

Question 8

This boy’s recurrent respiratory infections, history of foul-smelling stools that float, and small stature for his age are highly suggestive of cystic fibrosis. The resident flora of the lower respiratory tract in patients with this inherited disorder include Pseudomonas aeruginosa, which is also the most common cause of respiratory failure and death in cystic fibrosis. P. aeruginosa is an aerobic gram-negative, oxidase-positive, rod-shaped bacterium that produces pyocyanin, which gives it its blue-green color as seen in the microscopic analysis of this patient’s sputum culture. A major virulence factor of P. aeruginosa is exotoxin A, which ?

Answer 8

ADP ribosylates and inhibits elongation factor 2 in the host cell, thereby inhibiting protein synthesis.

Question 9

Toxins that block the release of neurotransmitters, as seen with Clostridium tetani and Clostridium botulinum, would cause symptoms of tetani or paralysis, respectively. In addition, these organisms would show up on microscopic analysis as gram-positive rods.

Toxic shock syndrome toxin-1 acts by directly binding to and activating MHC-II and T-lymphocyte receptors, and is secreted by Staphylococcus aureus, which would show up as gram-positive cocci in clusters on gram stain.

Streptococcus pneumoniae secretes an?

Answer 9

IgA protease that inactivates mucosal IgA antibodies to facilitate attachment to the host’s epithelial cells. It would appear microscopically as gram-positive, lancet-shaped diplococci. Bordetella pertussis secretes a pertussis toxin that inactivates Gi, resulting in an over-activation of adenylate cyclase which leads to an increased amount of cAMP. This pathogen is seen microscopically as a gram-positive coccobacillus.

Question 10

Pseudomonas aeruginosa is a common cause of repeated pneumonia in patients with cystic fibrosis and is known to produce exotoxin A, which ADP ribosylates and inhibits?

Answer 10

ribosomal elongation factor 2 in the host cell, thereby shutting down protein synthesis.

Question 11

This patient presents with a fever, small erythematous papules on the palms of her hands and raised lesions on her finger pads. She also has a history of mitral valve prolapse, and an eye examination shows white spots on the retina surrounded by hemorrhage. Together, these suggest a diagnosis of ?

Answer 11

subacute bacterial endocarditis, which is characterized by fever and signs of microemboli. Signs of microemboli include Osler nodes (tender raised lesions on finger and toe pads), Roth spots (round, white spots on the retina surrounded by hemorrhage; see arrow in the vignette image), and Janeway lesions (small, nontender, painless, erythematous lesions on the palm or sole). Splinter hemorrhages (small blood clots that run vertically under the fingernails) are also seen with microemboli. Another symptom of endocarditis is a new-onset heart murmur.

Question 12

Assessing these findings using the Modified Duke Criteria (which requires the presence of 5 of the criteria categorized as “minor criteria”), this patient’s diagnosis of subacute bacterial endocarditis can be confirmed: 1) predisiposing factor (history of mitral valves prolapse; 2) temperature >38C (patient’s temperature is 38.5C); 3) vascular phenomena (patient has signs of microemboli); 4) immunologic phenomena (patient has Osler nodes - tender, raised lesions); 5) microbiologic evidence (most likely organism tested in this question).

Viridans streptococci (eg, S. mutans, S. sanguinis, S. oralis, and S. mitis) are gram-positive cocci that grow in chains. They often cause ?

Answer 12

subacute bacterial endocarditis, particularly in the setting of mitral valve prolapse. They generally have low virulence but can form small vegetations on congenitally abnormal or damaged valves when seeded into the bloodstream. Complications from endocarditis include emboli and heart failure.

A popular mnemonic for infective endocarditis is “FROM JANE,” is described below:

Mnemonic: “FROM JANE”
Fever
Roth’s spots
Osler’s nodes (Osler = Ouch)
Murmur

Janeway lesions
Anemia
Nail hemorrhage (splinter hemorrhages)
Emboli

Question 13

Streptococcus agalactiae is known for causing meningitis in neonates. H. influenzae infection can result in a host of childhood illnesses. Staphylococcus aureus is associated with?

Answer 13

infective endocarditis (IE) in intravenous drug users. Streptococcus pyogenes infection can result in acute glomerulonephritis or rheumatic fever.

Question 14

Viridans streptococci such as S. sanguinis are the most common cause of subacute bacterial endocarditis in patients with damaged valves. Clinical signs include ?

Answer 14

retinal spots (Roth spots) and Janeway lesions (nontender lesions on the hands).

Question 15

This patient presents with recurrent bouts of dizziness, palpitations, and chest pain. He also has a history of a recent hiking trip in eastern Massachusetts. Together, these suggest a diagnosis of ?

Answer 15

Lyme disease. Lyme disease is caused by infection with the spirochete Borrelia burgdorferi and is transmitted by the bite of the Ixodes tick. Initially, the disease manifests with constitutional symptoms such as fever and malaise, as well as a rash surrounding the bite site. However, the bite site often goes unnoticed, and erythema chronicum migrans—the typical “bulls-eye” rash—is not necessarily present in every case.
Early disseminated Lyme disease manifests 4–6 weeks after the initial infection and is characterized by cardiac and neurologic abnormalities. Cardiac abnormalities include myocarditis, arrhythmias (eg, AV heart block), and conduction disturbances. The ECG in the stem shows no relationship between the P waves and QRS complexes, suggesting that the atria and ventricles are beating independently (AV dissociation). This is an example of third-degree AV heart block.

Question 16

Lyme arthritis is a late-stage finding, and occurs in about 60% of patients months to years later. The arthritis is associated with pain and swelling of large joints, most often in one or both knees. Lyme disease is most prevalent in states along the northeast Atlantic Coast, but cases have been reported throughout the United States.

Brugada syndrome is an inherited sodium channel abnormality that leads to ?

Answer 16

arrhythmias in young men. Chagas disease is endemic in South America, and can cause arrhythmias and dilated cardiomyopathy. Familial heart block is usually a second-degree rather than a complete block. Hypertrophic cardiomyopathy rarely causes distinctive EKG changes, except for hypertrophy and some repolarization changes.

Question 17

Lyme disease is a cause of heart arrhythmias in young people, and can present with ?

Answer 17

a third-degree AV block.

Question 18

The patient is a young immigrant from Mexico with a recent sick contact. The patient presents with fever, painful cheeks, and a bilaterally tender and swollen mandibular area, suggesting a likely diagnosis of parotitis. Given the acute onset and the fact that the patient is likely not vaccinated, the most likely diagnosis is mumps.

Although not often seen in the United States because of the measles/mumps/rubella (MMR) vaccine, mumps occasionally manifests in those who have not been vaccinated. Common complications associated with a mumps infection include ?

Answer 18

orchitis, parotitis, pancreatitis, and meningitis. Mumps is caused by an RNA paramyxovirus that replicates in the upper respiratory tract. Mumps is transmitted via respiratory droplets and direct contact. Treatment is usually limited to symptom management and relief (ie, pain medication).

Question 19

Hepatomegaly and splenomegaly are findings in infectious mononucleosis which are unlikely in our patient, given the absence of symptoms such as sore throat, lymphadenopathy, or fatigue. Papilledema is a finding consistent with?

Answer 19

increased intracranial pressure that can be seen in meningitis. Peripheral edema is a finding consistent with cellulitis; however, the absence of a rash makes this diagnosis unlikely in our patient.

Question 20

Complications of a mumps infection include?

Answer 20

parotitis, orchitis, pancreatitis, and meningitis. Because of routine measles/mumps/rubella vaccination, mumps is relatively uncommon in the United States.

Question 21

This patient presents with viral prodromal symptoms (ie, fever, headache, and diarrhea) that are followed by development of a reticular, maculopapular, eruptive, erythematous, and centrally distributed rash (like that shown in the image provided with the question). This description is classic for fifth disease, which is caused by parvovirus B19. The disease itself is?

Answer 21

self-limiting, requiring no treatment.
Parvovirus B19 infection is a pediatric illness common in children 3–12 years old. The rash is called erythema infectiosum and develops after fever has resolved as a bright, blanchable erythema on the cheeks (“slapped cheeks”) with perioral pallor. A more diffuse rash appears on the trunk and extremities and may wax and wane with temperature changes over 3 weeks. In adults, the disease more frequently causes arthralgia of the hands, wrists, knees, and ankles. It can be dangerous for patients with sickle cell anemia because the virus can infect erythroid precursors and inhibit bone marrow production of RBCs, resulting in profound anemia.

Question 22

Systemic corticosteroids are commonly used to treat asthma exacerbations and autoimmune diseases. This patient has no prominent pulmonary symptoms, and he lacks symptoms of arthritis, which would indicate an autoimmune disease. Topical corticosteroids may be helpful in treating?

Answer 22

allergic or inflammatory rashes, but the rash of erythema infectiosum is self-limited and would not respond to steroids. An oral or topical antibiotic would not be effective for this viral infection.

Question 23

Fifth disease (erythema infectiosum) is a self-limited condition caused by parvovirus B19. The associated rash, seen in children, is called erythema infectiosum and develops after fever has resolved. The rash is described as a?

Answer 23

centrally distributed, bright, blanchable, maculopapular, erythematous rash on the cheeks (“slapped cheeks”) with perioral pallor.

Question 24

You’ll need to piece this story together from several clues. Let’s start with the patient’s signs and symptoms. A productive cough, fever, dyspnea, and absent lung sounds in the lower lobe are clear indicators of a bilateral lower lobe pneumonia.

Next, look at her blood smear, which shows Howell-Jolly bodies. Howell-Jolly bodies are leftover nuclear remnants that are usually removed from RBCs in the spleen. Their presence indicates an?

Answer 24

absent or nonfunctioning spleen (asplenia). What diseases can cause the spleen to malfunction?

High on your differential must be sickle cell anemia, which is one of the most common hemoglobinopathies in African Americans. Sickle cell anemia can lead to autosplenectomy (spontaneous infarction of the spleen), and patients with asplenia are at increased risk for infection caused by encapsulated bacteria.

So you’re looking for an encapsulated bacterium that commonly causes community-acquired lobar pneumonia, especially in patients with asplenia. The answer? Streptococcus pneumoniae.

Question 25

S. pneumoniae colonizes the oropharynx by using surface protein adhesins to bind to epithelial cells. Secretory IgA prevents the subsequent migration of the organism to the lower respiratory tract by binding the bacteria to mucin with the Fc region of the antibody, enveloping the bacteria in mucus. Once trapped in mucus, the bacteria can be removed from airways by ciliated epithelial cells.

To fight back, S. pneumoniae secretes?

Answer 25

IgA protease and pneumolysin. IgA protease is an enzyme that cleaves IgA, thus allowing the bacteria to colonize mucous membranes. Pneumolysin is a cytotoxin similar to the streptolysin O in S. pyogenes. It binds cholesterol in the host cell membrane and creates pores that can destroy the ciliated epithelial cells and phagocytic cells.

Question 26

Polysaccharide capsules are the main virulence factor for encapsulated organisms because they provide resistance to phagocytosis, but they are not considered the primary factor responsible for colonization of the lower lungs. Phosphorylcholine on the bacterial cell wall binds to platelet-activating factor receptors on endothelial cells, leukocytes, platelets, tissue cells, lungs, and meninges. This action of phosphorylcholine allows bacteria to enter cells and facilitate the spread of disease, but it is not responsible for colonization of the lower lungs. Surface protein adhesion molecules mediate the initial colonization of the oropharynx; however, this is overcome by mucus secretion and ciliary clearance of the respiratory tract. Protein A is a ?

Answer 26

virulence factor associated with Staphylococcus aureus, and M protein is a virulence factor associated with Streptococcus pyogenes.

Streptococcus pneumoniae secretes IgA protease, which is its main colonizing factor and contains a polysaccharide capsule as its main virulence factor.

Question 27

This patient presents with the classic symptoms (fever, sore throat, anterior cervical lymphadenopathy, lack of cough) of streptococcal pharyngitis (strep throat). This is caused by ?

Answer 27

the catalase-negative, ß-hemolytic, bacitracin-sensitive Streptococcus pyogenes.

Catalase is an enzyme that catalyzes the breakdown of hydrogen peroxide into water. ß-Hemolysis occurs when there is complete lysis of RBCs in the media surrounding the bacterial colony (see image); this lysis is caused by the streptolysin exotoxin. Bacitracin is an antibiotic that is commonly used to characterize bacterial strains. Diagnosis is confirmed with a rapid strep test or with throat swab culture, and treatment is with penicillin.

Question 28

Catalase-negative, a-hemolytic, optochin-resistant describes Streptococcus viridans, which does not present with oropharyngeal exudates. Streptococcus pneumoniae is catalase negative, a-hemolytic, and optochin sensitive and, like S. viridans, does not cause oropharyngeal exudates. Streptococcus agalactiae is catalase negative, ß-hemolytic, and bacitracin resistant but is unlikely to be the cause of this patient’s infection because it typically affects neonates. Catalase-positive, coagulase-negative, novobiocin-sensitive describes ?

Answer 28

Staphylococcus epidermidis, which does not typically infect the upper respiratory tract.

Question 29

Streptococcus pyogenes is a?

Answer 29

group A β-hemolytic, catalase-negative, bacitracin-sensitive organism that causes pharyngitis.