Micro USMLE 8-28(12) (qmax 8/23 - 6-40)

Question 1

A 12-year-old boy with sickle cell disease presents to the emergency department with complaints of severe pain in the area of his right humerus. His temperature is 37.1°C (98.8°F), blood pressure is 100/60 mm Hg, pulse is 89/min, and respiratory rate is 22/min. Physical examination shows no other abnormalities. A radiograph is obtained that shows lytic changes and periosteal elevation in the middle and distal humeral shaft.

What is the most likely pathogen responsible for this patient’s condition?

Answer 1

Salmonella is the most common cause of osteomyelitis in patients with sickle cell disease. Sickle cell disease refers to any combination of Hb that includes HbS, whereas sickle cell anemia classically refers to homozygous HbS, and sickle cell trait refers to heterozygous HbS. The biochemical abnormality responsible for this condition occurs when glutamic acid, the 6th amino acid on the β-globin peptide chain, is replaced by valine (an E6V missense mutation). Therefore patients create HbS instead of HbA. At low oxygen levels, HbS aggregates and distorts red blood cells into a sickle shape. In the United States, 36 in 500 African Americans carry the sickle cell trait and 1 in 365 African Americans have sickle cell disease, making this ethnic group at highest risk for sickle cell disease.

Question 2

Historically, heterozygotes for HbS are believed to carry a survival advantage with protection from malaria. Patients with sickle cell anemia often suffer from aplastic crisis, splenic sequestration crisis, acute chest syndrome, and osteomyelitis. Aplastic anemia often arises from infection with parvovirus B19 (fifth disease). Children with this disease are also very susceptible to infection from encapsulated bacteria, as their spleen is often nonfunctional after the age of 4 years. Other common pathogens which cause osteomyelitis in sickle cell patients include?

Answer 2

Escherichia coli and Staphylococcus aureus.

Question 3

Pasteurella multocida is a cause of osteomyelitis in persons with a history of cat and dog bites or scratches. This gram-negative, non-spore-forming bacterial species commonly resides in domestic pet species, transferring to humans with an animal bite or scratch. Locally, the infection causes?

Answer 3

cellulitis or abscesses to develop in the area of injury. If untreated, it can become blood-borne, and infected persons can develop osteomyelitis or septic arthritis. These complications are more likely in cat bites, as cat bites tend to be smaller and more frequently go unnoticed.

Question 4

Candida albicans should be suspected in intravenous drug users who present with osteomyelitis. Candida is the most common fungal pathogen affecting humans. It is a fungus that can form pseudohyphae and true hyphae. One of the most common manifestations of Candida infection is as thrush, a creamy white oral plaque, in immunocompromised patients. It is also responsible for vulvovaginitis in females, often described as having a “cottage-cheese” consistency. Osteomyelitis due to Candida is rare in the general population; therefore, drug use should be suspected if a patient presents with osteomyelitis due to Candida.

Mycobacterium tuberculosis usually causes?

Answer 4

osteomyelitis from hematogenous spread in immunocompromised patients or in areas of endemic infection. Additionally, tuberculosis usually involves the vertebral bodies (Pott disease), not the humerus. Infection occurs by inhaling aerosolized particles of M. tuberculosis from an infected person. As this organism is slow growing, it can remain inert for many years before causing symptoms. Pott disease, also known as tuberculous spondylitis, is rare in developed countries, where antituberculous drugs are readily available. Patients present with a combination of osteomyelitis and arthritis involving more than one vertebra. They often have a history of untreated tuberculosis infection and complain of back pain for several weeks

Question 5

Neisseria gonorrhoeae is a rare cause of osteomyelitis in sexually active people, but is more likely to result in septic arthritis. This gram-negative, aerobic diplococcus can lead to pelvic inflammatory disease in females if left untreated, leading to infertility and tubal scarring. Other serious consequences include Fitz-Hugh and Curtis syndrome, in which the infection spreads from the reproductive organs to the liver capsule, causing acute perihepatitis.

Infection by Pseudomonas aeruginosa, like Candida spp., should be suspected in intravenous drug users who present with osteomyelitis. This gram-negative rod classically produces ?

Answer 5

a blue-green pigment with a sweet odor. Pseudomonas is an opportunistic pathogen seen in intravenous drug users, immunocompromised persons, and in burn wound sepsis. Osteomyelitis due to Pseudomonas can be blood-borne, from a systemic infection, or it may be contiguous, spread from penetrating wound trauma. Persons with a history of puncture wounds of the feet are at risk for Pseudomonas infection.

Question 6

Staphylococcus aureus is the most common cause of osteomyelitis in the general populaton but only the second most common cause of osteomyelitis in sickle cell patients, accounting for a quarter of cases. Salmonella is the most common cause of osteomyelitis in sickle cell patients. S. aureus is a gram-positive coccus often responsible for superficial skin infections and also causing joint infections after orthopedic procedures. S. aureus is often carried in the nares or perineum of indiviudals, sometimes leading to autoinoculation. The organism can be sensitive to methicillin or resistant to methicillin, making laboratory sensitivities often essential to its successful treatment.

Staphylococcus epidermidis, along with Staphylococcus aureus, can cause osteomyelitis in patients who have undergone prosthetic replacements. S. epidermidis is gram-positive and coagulase-negative, whereas S. aureus is gram-positive and coagulase-positive. S. epidermidis is found on laboratory cultures due to?

Answer 6

skin contamination, where it may reside. However, it is a major cause of infection in the immunocompromised and in patients with indwelling catheters. Its ability to form a biofilm on prosthetic devices makes it a possible cause of infection in those with joint replacements.

In patients with sickle cell anemia, consider both Salmonella and Staphylococcus aureus as causes for osteomyelitis. The lytic bone changes and periosteal elevation signifies an infection of bone and therefore osteomyelitis.

Question 7

This patient’s symptoms of chest pain that worsens with inspiration, intermittent fever, and hemoptysis—combined with his recent travel to a region in which tuberculosis (TB) is endemic—suggest a diagnosis of Mycobacterium tuberculosis infection. His chest x-ray with small nodules scattered in the right lung is indicative of TB. M. tuberculosis is an aerobic, acid-fast bacillus, which is revealed by?

Answer 7

Ziehl-Neelsen stain (see image). In addition to the symptoms seen in this patient, TB typically presents with night sweats and weight loss. TB can also cause pleural effusion, peritoneal ascites, and lymphadenitis. This patient likely has a pleural effusion, as indicated by the dullness to percussion, decreased breath sounds, and pleuritic chest pain on the right side. Patients with TB pleurisy also tend to have an exudate on pleural fluid analysis. (A diagnosis of TB can also be confirmed via polymerase chain reaction analysis from the pleural fluid).

Question 8

Congo red is used to visualize amyloid deposits. Giemsa stain is used to visualize Borrelia, Plasmodium, Trypanosoma, and Chlamydia organisms. India ink is used to visualize?

Answer 8

Cryptococcus neoformans. Periodic acid-Schiff stains glycogen and mucopolysaccharides.

Question 9

TB can manifest with fever, night sweats, weight loss, and blood in the sputum. Fibrocaseous cavitary lung lesions and pleural effusion can often be seen on x-ray of the chest. M. tuberculosis is initially detected with?

Answer 9

the acid-fast stain, Ziehl Neelson, but definitive diagnosis of infection with M. tuberculosis requires culture or molecular detection methods.

Question 10

The patient has fever, chills, and warmth and erythema over the dorsal lateral aspect of his foot (likely secondary to IV drug usage at this site).
X-ray shows lytic destruction, as indicated by the arrows, of multiple bones in the foot that are worst in the third through fifth metatarsal heads and corresponding proximal phalanges. He also has increased C-reactive protein (CRP) and erythrocyte sedimentation rate, which are non-specific markers for inflammation. All of these findings together suggest osteomyelitis. Staphylococcus aureus is the most common cause of osteomyelitis in the general population and should be assumed if no other information about the patient is given. Staphylococcus aureus is?

Answer 10

Gram-positive, catalase positive, and coagulase positive. Among patients who use illicit intravenous drugs, Pseudomonas aeruginosa should also be considered as a highly likely candidate, but it is still less common than S. aureus.

Question 11

Mycobacterium tuberculosis, osteomyelitis is most common in the vertebrae and is detected with an acid-fast stain and culture. It is associated with a comparatively slow onset of illness, often with systemic signs and symptoms, and a positive tuberculin skin test.

Neisseria gonorrhoeae, which are Gram-negative, diplococci, aerobic, is more likely to cause septic arthritis than osteomyelitis. Septic arthritis does not usually affect the feet but rather one large joint. It is usually associated with younger, healthy, sexually active patients

Salmonella species, which are ?

Answer 11

Gram-negative, lactose non-fermenter, oxidase negative, positive hydrogen sulfide (H2S) production is a common cause of osteomyelitis in patients with sickle cell disease, but rare otherwise.
Coagulase-negative Staphylococcus species, which are Gram-positive, catalase positive, and coagulase negative organisms, are suspected in patients with a history of joint replacement or other device-associated infections.

Question 12

Staphylococcus aureus is the most common cause of osteomyelitis overall. Patients have non-specific infectious symptoms (fever, chills, malaise) bone pain. In addition, they may often have an?

Answer 12

increased C-reactive protein (CRP) and erythrocyte sedimentation rate which may depict lytic destruction of bone.

Question 13

This patient’s symptoms of high fever, dysphagia, inspiratory stridor, and respiratory distress are consistent with a diagnosis of epiglottitis. The x-ray (see image) shows thickening of the epiglottis (“thumb sign” indicated by arrow in the image) and aryepiglottic folds, which are pathognomonic for infection with Haemophilus influenzae. Although other organisms can cause epiglottitis, H. influenzae more typically presents in young children and is rapidly progressive with a high risk of airway obstruction. The vaccination against H. influenzae targets?

Answer 13

polyribosylribitol phosphate (type b polysaccharide capsule), which is its main virulence factor.

Question 14

M protein is associated with Streptococcus pyogenes; no vaccine against this protein has been developed. IgA protease is a virulence factor of H. influenzae but is not the target of vaccinations. Protein A is a surface protein produced by?

Answer 14

Staphylococcus aureus, which is not associated with epiglottitis. Type Vi polysaccharide capsule is a target of vaccination against Salmonella typhi.

Question 15

A child with a high fever, dysphagia, difficulty breathing, and inspiratory distress should be evaluated for epiglottitis, which is most often caused by H. influenzae type B, a gram-negative coccobacillus. Pathognomonic findings on an x-ray include thickening of the epiglottis (thumbprint sign) and aryepiglottic folds. The vaccine against H. influenzae type B produces antibodies against ?

Answer 15

the polyribosylribitol phosphate that comprises the type B capsule.

Question 16

This patient’s changed cardiac murmur in the setting of low-grade fever, malaise, and weight loss is highly suggestive of subacute bacterial endocarditis, which is most often caused by viridans streptococci. These organisms are ?

Answer 16

gram-positive, optochin-resistant, α-hemolytic cocci that grow in chains (see image).
Fifty to sixty percent of cases of endocarditis affecting native (not prosthetic) valves are caused by these bacteria. Viridans streptococci usually attack valves that have previous damage. As in this patient with a history of mitral valve prolapse, in the general population the mitral valve is the most commonly damaged valve. Physical exam findings that would support the diagnosis of endocarditis also include nail-bed splinter hemorrhages, Osler nodes (tender raised lesions on the fingers or toe pads due to immune complex deposition), and Janeway lesions (small, painless, erythematous lesions on the palms or soles).

Question 17

Many bacteria and viruses affect the cardiovascular system, but their association with subacute bacterial endocarditis is not as direct as that of viridans streptococci. Staphylococcus epidermidis organisms are described as gram-positive, catalase-positive, coagulase-negative cocci in clusters. These bacteria are often the cause of tricuspid endocarditis in patients with a history of intravenous drug use. Gram-positive, catalase-positive, coagulase-positive cocci in clusters is a description of Staphylococcus aureus, which is also associated with endocarditis and a history of intravenous drug use.

Gram-positive, optochin-sensitive, catalase-negative, coagulase-negative, α-hemolytic cocci growing in pairs is the characteristic appearance of ?

Answer 17

Streptococcus pneumoniae, which is the most common cause of meningitis in adults, otitis media in children, bacterial pneumonia, and sinusitis. Coxsackievirus B, a positive-stranded RNA enterovirus, causes viral myocarditis. Mycobacterium tuberculosis—appearing as weakly gram-positive, acid-fast rods that grow in straight or branching chains—causes tuberculosis.

Question 18

Subacute bacterial endocarditis occurs in individuals who have pre-existing valvular disease. Symptoms include low-grade fever, malaise, and weight loss over a period of weeks to months. The causative bacteria are most likely ?

Answer 18

viridans streptococci, which are gram-positive, optochin-resistant, α-hemolytic cocci.

Question 19

This patient presents with fever, as well as coughing, wheezing, and hemoptysis. Given the finding of a cavitary lung mass and his history of sarcoidosis, Aspergillus fumigatus has likely infected the pre-existing lung cavities to form fungal balls, also called aspergillomas.

Under the microscope, A. fumigatus appears as a nondimorphic mold with ?

Answer 19

septate hyphae that branch at a V-shaped 45-degree angle (as seen in the image). In patients with significant hemoptysis, treatment is surgical removal of the aspergilloma. Otherwise, itraconazole may be prescribed. A. fumigatus also produces aflatoxins that, when ingested in sufficient quantities, can cause hepatocellular carcinoma.

Question 20

Squamous cell carcinoma of the lung is strongly associated with tobacco smoking and is unlikely to present with fever and air crescents on imaging.
Although the symptoms of aspergillosis can resemble infection with Mycobacterium tuberculosis, which is cultured on Lowenstein-Jensen agar, this patient does not appear to have been exposed to tuberculosis.

Mucor species are molds with irregular nonseptate hyphae that branch at irregular angles, typically >90 degrees. Mucor infection is almost always associated with ?

Answer 20

an immunocompromised state, such as diabetic ketoacidosis in patients with diabetes.
Sporothrix is an elliptical budding yeast that is visible in pus and can cause an ascending lymphadenitis with cutaneous manifestations. It does not typically cause respiratory or systemic symptoms.

Question 21

Aspergillus fumagatus is a mold with septate hyphae that branch at V-shaped 45-degree angles. A. fumigatus infection commonly causes?

Answer 21

aspergillomas (“fungus balls”). Clinical features of invasive aspergillosis include fever, chills, hemoptysis, shortness of breath, headaches, and occasionally, chest pain.

Question 22

The patient is brought to the ED by her mother after exhibiting strange behavior and suffering amnesia. Based on her clinical presentation, lumbar puncture, and MRI, this patient most likely has encephalitis due to herpes simplex virus (HSV) type 1. The symptoms of HSV-1 encephalitis can range from amnesia and behavioral changes (eg, hypomania) to Klüver-Bucy syndrome (characterized by inappropriate sexual behaviors and mouthing of objects). Later stages of the disease can include?

Answer 22

coma and death.
On the lumbar puncture, viral meningitis should produce:

Increased lymphocytes
Normal to increased protein
Normal to decreased glucose
Elevated numbers of RBCs might also be noted, due to hemorrhage from the necrosis of brain tissue. CT or MRI of the brain typically demonstrates focal pathology in the temporal lobe (as indicated by arrows in this image).

Question 23

HSV-1 is the most common cause of sporadic encephalitis. HSV-2 is more likely to cause a meningitis, which is distinguished from encephalitis by the absence of significant cognitive changes. Other diseases caused by HSV-1 include?

Answer 23

gingivostomatitis (with lesions around the lips and gums), keratoconjunctivitis, and herpes labialis. HSV-2 is associated with herpes genitalis and neonatal herpes.

Question 24

Coxsackievirus is a common cause of viral meningitis and dilated cardiomyopathy, but it doesn’t demonstrate the same effects on the temporal lobe.
Streptococcus pneumoniae is the most common cause of otitis media, as well as meningitis overall, but this immunized patient’s lumbar puncture do not reflect a bacterial cause (high protein, low glucose, and neutrophil prominence).
The lumbar puncture and immunization status also helps to ?

Answer 24

rule out an infection with Haemophilus influenzae, which can cause epiglottitis in children.
Toxoplasma gondii is a parasite that can lead to chorioretinitis, especially in the gestational period. However, neuroimaging would be expected to show distinctive ring-enhancing lesions.

Question 25

Herpes simplex encephalitis due to HSV-1 can affect any age group but is most often seen in persons under age 20 or over age 40. Symptoms can include headache and fever for up to 5 days, followed by personality and behavioral changes, seizures, hallucinations, and altered levels of consciousness. Lumbar puncture results are often consistent with?

Answer 25

viral meningitis, although MRI may be normal early in the course of illness. Temporal lobe involvement and early involvement of white matter are typical.

Question 26

The patient’s medical history is the key to identifying the cause of his current symptoms. Measles, which is caused by ?

Answer 26

the measles virus, typically manifests with a blanching red rash that starts on the face, then moves down to the trunk and limbs. The rash is often raised and accompanied by fever. The rash mostly resolves in the same way in which it appeared. This history may not be obtained in every patient.
Subacute sclerosing panencephalitis (SSPE) is a rare complication of measles that occurs within a range of 2–10 years after the initial infection and is initially characterized by personality changes, lethargy, difficulty in school, and odd behavior. It can progress to dementia, severe myoclonic jerking, and then to eventual flaccidity and decorticate rigidity. SSPE is always fatal, with most patients living 1–3 years after diagnosis. No cure for SSPE exists. Patients with SSPE often have high titers of measles antibodies in their serum and CSF, which can be detectable as oligoclonal bands.

Question 27

Like rubeola, roseola (HHV6) can cause a high fever and rash, but there are no long-term neurologic sequelae from infection.
Parvovirus similarly manifests with a rash on the face but also does not have any long-term neurologic complications.
Streptococcus pyogenes can cause ?

Answer 27

scarlet fever and a desquamating rash—along with long-term complications like glomerulonephritis and rheumatic fever—but rarely causes long-term neurologic problems.

Question 28

Rubella can cause ?

Answer 28

a rash and cervical lymphadenopathy, but does not have any long-term neurologic sequelae.

Question 29

Measles virus (rubeola) is capable of causing subacute sclerosing panencephalitis (SSPE), which results in ?

Answer 29

dementia, myoclonus, and personality changes, striking 2–10 years after initial infection. Very few cases are seen in the U.S. due to a nationwide vaccination program.