Micro USMLE 8-28(14) (qmax 8/23 - 6-40)

Question 1

This homeless patient presents with pain and swelling in his right upper arm in addition to diffuse soft tissue and bone inflammation. The image shows a bone abscess with polymorphonuclear leukocytes and foci of degraded bone collagen. These findings along with the blood culture suggest a likely diagnosis of ?

Answer 1

osteomyelitis.
There are many risk factors for osteomyelitis, or inflammation of the bone or bone marrow. Four of the possible answer are risk factors for osteomyelitis, but you can rule out sickle cell disease, which is associated with Staphylococcus (a gram-positive coccus) or Salmonella (a gram-negative but oxidase-negative rod).

Question 2

Miliary tuberculosis (which can be spread by close living quarters), multiple sexual partners, and intravenous drug use (IVDU) are all commonly seen in the homeless population. Miliary tuberculosis is caused by infection with Mycobacterium tuberculosis, which doesn’t Gram stain, and this patient’s lab results show a gram-negative organism. Having multiple sex partners puts a person at increased risk of sexually transmitted infections, such as Neisseria gonorrhoeae, which fits the description of a gram-negative, oxidase-positive bacterium. It’s a diplococcus, however, not a rod.

So that leaves IVDU, and such patients with osteomyelitis are most likely to be infected with?

Answer 2

S. aureus (a gram-positive coccus), Candida (a yeast), or Pseudomonas aeruginosa (a gram-negative, oxidase-positive rod.) P. aeruginosa enters the body via the needle puncture.

Finally, if you choose corticosteroid use, you are probably thinking about the association between these medications and osteoporosis (rather than osteomyelitis).

Question 3

Intravenous drug use is a major risk factor for osteomyelitis caused by?

Answer 3

Pseudomonas.

Question 4

This child presents with erythematous tonsils, petechiae and macules on her hard and soft palates, and erythematous patches on her trunk, axillae, and earlobes. In the context of her recent sore throat and fever, she most likely has scarlet fever (also called scarlatina) after an episode of pharyngitis.
Scarlet fever is caused by ?

Answer 4

Streptococcus pyogenes (also known as group A streptococci) strains, which produce erythrogenic toxins that stimulate lymphocyte blastogenesis, potentiate endotoxin-induced shock, and induce fever. Some strains produce pyrogenic exotoxins that are superantigens, stimulating a large portion of T lymphocytes, leading to an increased production of cytokines and an inflammatory response.

Question 5

The rash of scarlet fever is made up of many 1- to 2-mm papular lesions, making the affected area rough in texture. Papules are erythematous, but blanch with pressure. Lesions first appear on the neck, around the ears, or in the axillae and then spread to the trunk. The rash spares the face, but facial flushing with circumoral pallor, strawberry tongue (a red and bumpy surface that may be covered with a white coating (see image), and petechiae on the soft and hard palates are all features of the disease. Administration of antibiotics for ?

Answer 5

streptococcal pharyngitis can prevent the development of scarlet fever.

Question 6

Measles virus can also cause oral lesions, but those lesions (called Koplik spots) are white instead of red, and measles does not usually cause a prodrome of sore throat without a cough. Varicella zoster causes a vesicular rash and fever, not a macular rash. Rubella causes?

Answer 6

fever, sore throat, and macular rash but does not produce oral lesions. Staphylococcus aureus causes a variety of infections, including toxic shock syndrome and staphylococcal scalded skin syndrome, both of which present with an exfoliative, desquamating rash and can cause severe hypotension.

Question 7

Scarlet fever is caused by?

Answer 7

Streptococcus pyogenes (group A streptococci). It is mediated by a pyrogenic exotoxin, and symptoms include high fever and a diffuse, erythematous, rough-textured rash. Scarlet fever can be prevented by administration of antibiotics for streptococcal pharyngitis.

Question 8

The concern for this 2-month-old infant is meningitis, which can be caused by bacteria, viruses, or fungi. The classic symptom triad is fever, headache, and nuchal rigidity. In an infant with an open fontanelle, there will be no nuchal rigidity, but the open fontanelle will appear to bulge. Other symptoms such as rash and photophobia are also commonly present. Inference from the history, such as severe crying in a normally quiet baby, may indicate headache or muscle stiffness.
A lumbar puncture and cerebrospinal fluid (CSF) analysis is essential in determining the cause. Classic CSF findings for bacterial meningitis are elevated opening pressure, elevated levels of polymorphonuclear leukocytes, elevated protein levels, and decreased glucose levels, as seen in this patient. In newborns age 0–6 months old, the three most common causes of meningitis are all bacterial and include?

Answer 8

group B Streptococci, Escherichia coli, and Listeria monocytogenes. Culture and microbiology analysis can differentiate among these species.

In newborns and babies, the meningitis symptoms of fever, headache, and neck stiffness may be absent or difficult to notice. However, the mortality rate for untreated meningitis is high, and patients can quickly deteriorate. Therefore, empiric treatment should be initiated prior to obtaining the results of CSF analysis.

Question 9

Although Haemophilus influenzae was previously an important cause of bacterial meningitis, the HiB vaccine has dramatically reduced its incidence.
Cryptococcus neoformans causes meningitis in immunocompromised patients and would feature elevated lymphocytes rather than neutrophils.
Cytomegaloviruses similarly can cause?

Answer 9

meningitis in immunocompromised patients and, like enteroviruses, features a classic “viral meningitis” lumbar puncture: normal glucose and protein, with elevated lymphocytes.
Neisseria meningitidis, is more common in patients between 6 and 60 years old, not in infants.

Question 10

Group B streptococci, E. coli, and Listeria monocytogenes are the most common causes of meningitis in children 0–6 months old. In newborns and babies, the meningitis symptoms of fever, headache, and neck stiffness may be absent or difficult to notice. Definitive diagnosis can be made based on CSF analysis, with meningeal inflammation demonstrated by?

Answer 10

increased WBC count, elevated protein level, and low glucose level in the CSF.

Question 11

The child’s presentation of yellow, perioral crusted vesicles is consistent with a diagnosis of non-bullous impetigo. Staphylococcus aureus and Streptococcus pyogenes are the two most common causes of impetigo. Because there is no clear consensus for which of the two is more common, identification should be based on additional information.
Microscopy of the child’s exudate shows clusters of gram-positive cocci which identify S. aureus as the culprit. S. aureus is coagulase-positive. Coagulase enables?

Answer 11

the conversion of fibrinogen to fibrin. The formation of fibrin clots around S. aureus can lead to abscess formation. Exfoliative toxin A released by S. aureus disrupts cell adhesions in the epidermis, thereby causing an eruption of vesicles on the face. These vesicles later turn into pustules with a characteristic honey-colored crust (see example below).

S. aureus is beta-hemolytic, not alpha-hemolytic. Novobiocin sensitivity is specific for Staphylococcus epidermidis. PYR positivity and bacitracin sensitivity are specific to Streptococcus pyogenes. While it’s a common cause of impetigo, S. pyogenes is not consistent with gram-positive cocci in clusters described in the microscopic findings.

Question 12

S. aureus and S. pyogenes are both common causes of non-bullous impetigo. Impetigo is characterized by vesicles that become pustules and develop?

Answer 12

yellow, crusted lesions, often occurring around the mouth.

Question 13

This young patient who is likely unvaccinated (never seen a doctor) and presents with 1 week of catarrhal symptoms (malaise, runny nose, cough) along with post-tussive vomiting most likely has pertussis, or “whooping cough.” Pertussis is caused by the gram-negative coccobacillus Bordetella pertussis. The characteristic presentation of pertussis allows for diagnosis to be made clinically. Nevertheless, culture of the nasopharynx can be useful when the diagnosis is uncertain. Difficulties culturing B. pertussis led to the development of?

Answer 13

Bordet-Gengou agar, which is made with blood, potato extract, glycerol, and antibiotics. Regan-Lowe agar, a charcoal blood agar, may also be used to culture B. pertussis.

Pertussis was predominantly a childhood disease before the widespread use of immunizations, and now mostly occurs in unvaccinated children and older patients with waning immunity. It begins with upper respiratory symptoms (eg, malaise, rhinorrhea, congestion) and may progress to episodes of intense coughing that begin with a “whooping” inhale and result in vomiting. Lung auscultation and chest radiography are usually normal. Lymphocytosis induced by pertussis toxin is common and can cause fever. Treatment involves antibiotic therapy with a macrolide, such as azithromycin, as well as prophylaxis for all close contacts regardless of vaccination status.

Question 14

MacConkey agar contains bile salts that inhibit the growth of gram-positive organisms. MacConkey agar is optimized for the culture of lactose-fermenting enteric bacteria.
Thayer-Martin agar, also known as Thayer-Martin VCN medium, is a blood agar supplemented with antimicrobials (VCN = vancomycin, colistin [polymyxin], and nystatin). It restricts the growth of gram-positive organisms and gram-negative organisms other than Neisseria spp, making it ideal for the culturing of ?

Answer 14

N. gonorrhea and N. meningitidis.
Haemophilus influenzae is a fastidious organism and requires both factors V and X to grow; it can be grown on blood agar alongside Staphylococcus aureus, which facilitates release of additional factor X.
Löffler’s agar contains horse serum, meat infusion, and dextrose and selectively grows the genus Corynebacterium.

Question 15

Bordetella pertussis causes pertussis (“whooping cough”). When diagnosis is uncertain, it can be cultured from the nasopharynx onto?

Answer 15

Bordet-Gengou agar or Regan-Lowe agar.

Question 16

This patient presents with a moderate pulmonary infection with extrapulmonary symptoms, including verrucous skin lesions and bone lytic lesions, after an outdoors trip to the Great Lakes. This clinical scenario is suggestive of systemic mycoses, specifically Blastomycosis, which occurs after inhalation of ?

Answer 16

the dimorphic fungus Blastomyces dermatitidis. The pathogen is endemic to the Midwest regions of the United States, including the Great Lakes.

Question 17

The lungs are the most common site of infection, and the disease has a tendency to spread via the blood, often affecting the skin and bones. The skin is the most common site of extrapulmonary infection, followed by the bone. Biopsy specimen of affected areas shows the characteristic yeast, which is?

Answer 17

thick walled and spherical, producing single buds with a broad base of attachment. Blastomycosis often presents with flu-like symptoms, fevers, chills, cough, and pleuritic chest pain. It may develop into a chronic pulmonary condition. Notably, disseminated infection can often develop in both immunocompetent and immunocompromised patients, although the severity is greater in those who are immunocompromised. Moderate pulmonary disease and mild disseminated disease, as seen in this patient, may be treated with oral itraconazole. Severe pulmonary disease, moderate disseminated disease, and disease in immunocompromised patients may be treated with intravenous amphotericin B.

Question 18

The finding of oval yeast cells within macrophages is typical of histoplasmosis, which is also endemic to the Midwest United States and causes pulmonary disease. In contrast, disseminated histoplasmosis would be rare in an individual who is not immunocompromised.

The finding of thick-walled spherules filled with endospores is typical of coccidioidomycosis, a systemic mycosis that is endemic to the Southwestern United States. Coccidioidomycosis mostly causes pulmonary infection. It would not cause the verrucous skin lesions and bone involvement seen in this patient.

The finding of budding yeast with “captain’s wheel” formation (cells covered in budding blastoconidia) is characteristic of ?

Answer 18

paracoccidioidomycosis, a systemic mycosis that is endemic to Latin America. This patient has had recent exposure to the Midwest Great Lakes area, not Latin America.

The finding of strands and spores with “spaghetti and meatballs” appearance is typical of tinea versicolor, a cutaneous mycosis caused by Malassezia species. Affected patients have a fungal infection limited to the skin; hypopigmented patches are common. This patient has productive cough, verrucous skin lesions, and bone lesions, suggestive of a disseminated infection; this presentation would not be explained by tinea versicolor infection.

Question 19

Blastomycosis is endemic to the Midwest regions of the United States, including the Great Lakes. The skin is the most common site of extrapulmonary infection, followed by the bone. Biopsy specimens of affected areas show the characteristic yeast, which is thick walled and spherical, producing ?

Answer 19

single buds with a broad base of attachment.

Question 20

This child presents with an upper-respiratory-like illness of 2-week duration and spontaneous coughing fits followed by emesis. Her negative history for immunizations should raise suspicion for pertussis (second stage), an infection caused by Bordetella pertussis.
Infection with B. pertussis, a gram-negative coccobacillus, presents in three clinical stages:

1. Catarrhal stage, lasting 1–2 weeks, characterized by symptoms that resemble an upper respiratory infection.
2. Paroxysmal stage, lasting 2–8 weeks, characterized by paroxysms of coughing, an inspiratory whoop, and posttussive vomiting.
3. Convalescent stage, in which the cough subsides over several weeks to months.
   Pertussis is seen mainly in infants and young children, but is diagnosed infrequently in the United States as a result of the nearly universal use of the pertussis vaccine. B. pertussis can be grown on ?

Answer 20

Bordet-Gengou agar and produces an exotoxin that ADP-ribosylates and inactivates Gi proteins.

Question 21

Legionella pneumophila is grown on charcoal yeast extract medium buffered with iron and cysteine. Patients with this infection will have?

Answer 21

gastrointestinal symptoms along with mild respiratory symptoms. Haemophilus influenzae is grown on chocolate agar containing hemin (factor X) and nicotinamide adenine dinucleotide (factor V). It is associated with bacterial meningitis, epiglottitis, pneumonia, empyema, pericarditis, bacteremia, and septic arthritis. Corynebacterium diphtheriae is grown on Loeffler medium which manifests most commonly as a sore throat, malaise, cervical lymphadenopathy, and low-grade fever.

Question 22

Neisseria gonorrhoeae, which is grown on Thayer-Martin agar, is a common sexually transmitted disease. In a female patient, it causes?

Answer 22

typical findings of cervicitis, including vaginal pruritus and a mucopurulent cervical discharge.

Question 23

Bordetella pertussis infection begins as an upper-respiratory-like infection and progresses to paroxysms of coughing, an inspiratory whoop, and posttussive vomiting. This pathogen is grown on?

Answer 23

Bordet-Gengou agar.