Micro USMLE 8-29(18) (qmax 8/24 1-31)

Question 1

This 1-year-old is brought in by her mother due to her fussiness and refusal to feed. Her fever and the cerebrospinal fluid findings (decreased glucose level, increased WBC count, elevated protein level, and opening pressure >180 mm H2O) likely indicate meningitis of bacterial origin. One of the most common causes of meningitis in the 1-year-old age group is ?

Answer 1

Haemophilus influenzae type B, but only in the unimmunized population.
The incidence of H. influenzae meningitis has greatly declined with the development of a highly effective conjugate vaccine. The other common causes in this age group include Streptococcus pneumoniae, Neisseria meningitidis, and the enteroviruses. This table classifies the main causes of meningitis by age.

Question 2

The incidence of H. influenzae meningitis has greatly declined with the development of a highly effective conjugate vaccine. The other common causes in this age group include Streptococcus pneumoniae, Neisseria meningitidis, and the enteroviruses. This table classifies the main causes of meningitis by age.

Clostridium botulinum causes botulism, which manifests with constipation followed by generalized muscle weakness, feeding difficulties, drooling, anorexia, irritability, and a weak cry.
Group B Streptococcus (GBS) is a common cause of ?

Answer 2

meningitis in children younger than 3 months of age.
Herpes simplex virus has been shown to cause meningitis in infants, but it is not a common source of the infection in this age group.
Listeria monocytogenes is a common cause of meningitis in newborns and in older adults >60 years old, but not in the 1-year-old age group.

Question 3

A common cause of bacterial meningitis in unimmunized 1-year-old children is ?

Answer 3

Haemophilus influenzae. Cerebrospinal fluid analysis in such cases would show decreased glucose levels, increased WBC counts, elevated protein levels, and increased opening pressure.

Question 4

The patient presents with dyspnea, systolic heart failure, and an enlarged cardiac silhouette on x-ray following a viral illness. These are all signs of dilated cardiomyopathy. Dilated cardiomyopathy can be caused by ?

Answer 4

coxsackievirus B, resulting in congestive heart failure (CHF). CHF shows eccentric hypertrophy, in which sarcomeres are added in series.
Elevated B-type natriuretic peptide is highly sensitive for CHF, and an S3 heart sound is highly specific for CHF. Other causes of dilated cardiomyopathy include alcohol or cocaine abuse, pregnancy, inherited genetic abnormalities, hemochromatosis, and doxorubicin. Notably, Trypanosoma cruzi, the causative agent of Chagas disease, is another infectious cause of CHF, although it is chronic in onset.

Question 5

oronavirus is responsible for the common cold, severe acute respiratory syndrome (SARS), and Middle East respiratory syndrome (MERS). Parainfluenza virus is a paramyxovirus that causes croup, while Streptococcus pyogenes is responsible for rheumatic fever. Streptococcus viridans is associated with?

Answer 5

subacute bacterial endocarditis, while Treponema pallidum causes syphilis.

Question 6

Dilated cardiomyopathy causes systolic dysfunction of the heart, and results in CHF with symptoms that include an S3 heart sound and dyspnea. Infectious agents leading to this disease include?

Answer 6

coxsackievirus B and Trypanosoma cruzi.

Question 7

This patient presents with a painless, growing mass that periodically exudes a yellow discharge. In the context of his oral surgery 1 month earlier, his symptoms suggest a related dental infection caused by Actinomyces israelli, an obligate anaerobe that is part of the normal oral flora.
The genus Actinomyces is closely related to the genus Nocardia. Both are unique among bacteria in that they exhibit a branching, rod-like morphology. A major difference between the two genera is that Nocardia organims are?

Answer 7

weakly acid-fast positive, whereas Actinomyces organisms are acid-fast negative. The image of the modified acid-fast stain shows no acid-fast bacilli, helping to exclude Nocardia as the cause of the infection. The large blue regions in the image are sulfur granules that are made up of mass growth of the actinomycetes.

Question 8

On Gram staining and microscopic examination, A israelli appear as gram-positive branching rods with clustering that froms “sulfur granules” in the thick yellow exudate (as described previously). Trauma associated with dental work often leads to the invasion of A israelli into the cervicofacial area. Infection caused by this organism typically presents as a chronic, slowly progressing mass that eventually evolves into a draining sinus tract. First-line treatment involves administration of penicillin G, an antibiotic that inhibits?

Answer 8

transpeptidase cross-linking and thus prevents cell wall synthesis, and surgical debridement.

Question 9

Sulfonamides, which are first-line treatment for Nocardia infections, act by blocking bacterial nucleotide synthesis. Macrolides exert their antibacterial effect by binding to the 50S ribosomal subunit, but they lack anaerobic coverage. Amphotericin B, which is used to treat ?

Answer 9

systemic mycoses, binds ergosterol, forming pores in the membrane. Azoles, which are also used to treat fungal infections, work by inhibiting ergosterol synthesis.

Question 10

Actinomyces israelii is a gram-positive, branching rod endogenous to the oral flora. Trauma from dental work often leads to inoculation of A israelii into cervicofacial areas, which can cause oral and facial abscesses that release yellow sulfur granules in the pus. First-line treatment includes administration of penicillin G, which acts by?

Answer 10

blocking bacterial cell wall synthesis by inhibiting transpeptidase cross-linking, and surgical debridement

Question 11

Based on the patient’s presentation and his employment history, he appears to be suffering from adult botulism. Adult botulism is characterized by descending paralysis, which typically begins with the development of diplopia and dysphagia. This is followed by the development of general muscle weakness, respiratory muscle failure, and even death. The organism responsible for this disease is Clostridium botulinum, and it can be found in contaminated homemade canned goods and smoked fish. C. botulinum produces a toxin that cleaves synaptobrevin, preventing release of acetylcholine at the presynaptic nerve terminals of ?

Answer 11

the neuromuscular junction , which can affect both nicotinic and cholinergic nerve terminals.

C. botulinum is a gram-positive, rod-shaped, anaerobic, spore-forming organism. The spores are very heat resistant, and improperly canned foods may not reach the temperature necessary to destroy these spores. Consequently, the organisms produce botulinum toxin, which can cause food-borne botulism when the toxin is ingeste

Question 12

nfant botulism, which causes floppy baby syndrome, is caused by the ingestion of C. botulinum spores, which colonize the infant’s gastrointestinal tract and release the toxin. Infants can become infected by the spores when they consume raw honey or ingest environmental soil/dust.

Muscle fibers can be affected in polymyositis and dermatomyositis, which manifest with muscle weakness, tenderness, and increased creatine kinase.
Peripheral nerves can be affected in?

Answer 12

Guillain-Barré syndrome, which manifests with an ascending symmetric paralysis with hyporeflexia, following an upper respiratory infection or gastrointestinal infection.
The anterior horn of the spinal cord can be affected by poliovirus, which can cause asymmetric proximal muscle weakness and lower motor neuron signs with a normal sensory exam.
Individuals with a brainstem infarct/hemorrhage can present with ipsilateral cranial nerve findings and contralateral sensory loss and hemiplegia.

Question 13

Botulism in adults is characterized by the 4 D’s: Diplopia, Dysarthria, Dysphagia, and Dyspnea. It is caused by the heat labile toxin produced by Clostridium botulinum, which blocks the release of acetylcholine at the ?

Answer 13

neuromuscular junction, resulting in a descending flaccid paralysis.

Question 14

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

S. pneumoniae is the most frequent cause of?

Answer 14

community-acquired and lobar pneumonias.
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 viruses and bacteria such as Mycoplasma pneumoniae and Chlamydia pneumoniae, which do not appear on Gram stain.

Question 15

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

Answer 15

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

Question 16

This patient has the classic presentation for diphtheria, a grayish white pseudomembrane on the pharynx or tonsils . Other symptoms of diphtheria, which is caused by the gram-positive rod Corynebacterium diphtheriae, include sore throat, malaise, and cervical lymphadenopathy. Fever is usually mild or absent. The pseudomembrane associated with this infection should not be disrupted to prevent increased absorption of the lethal exotoxin produced by this organism.
Diphtheria is rarely observed in vaccinated populations but is endemic to certain parts of the world. Moreover, cases may be seen in?

Answer 16

any developing country. Löffler medium and tellurite agar can be used in conjunction for culture and subsequent diagnosis of C. diptheriae infection. Löffler medium preferentially grows C. diptheriae, which appear as minute cream-colored colonies. In the presence of all Corynebacteria (including C. diphtherieae), tellurite agar will grow gray to black colonies as a result of potassium tellurite being reduced to metallic tellurite. Tinsdale agar can be used to differentiate C. diphtheriae from other Corynebacterium spp by the formation of brown to black colonies.

Question 17

Pertussis is caused by Bordetella pertussis and is grown on Bordet-Gengou agar; however, it does not present with pseudomembranes. Haemophilus influenzae is cultured on agar supplemented with factors V and X, also known as chocolate agar. H. influenzae can cause meningitis, epiglottitis, and otitis media; however, none of these infections present with pseudomembranes. Neisseria gonorrhoeae is grown on?

Answer 17

Thayer-Martin agar and causes gonorrhea but does not present with pseudomembranes. Fungal pathogens can be grown on Sabouraud agar. Fungal infections of the oropharynx include candidiasis, infection with Candida albicans; however, candidiasis does not usually present with systemic findings if the infection is localized.

Question 18

A patient with pharyngitis and a pseudomembrane on the pharynx is likely infected with C. diphtheriae, which is best cultured on?

Answer 18

a Löffler medium in combination with tellurite agar. Other symptoms of infection include a sore throat, cervical lymphadenopathy, and low-grade fever.

Question 19

This patient with night sweats and weight loss in the context of IV drug abuse and a low CD4 count, should raise a high suspicion for an immunocompromised state associated with HIV.
Candida stomatitis (often called thrush) or oral candidiasis and esophagitis is an opportunistic infection caused by Candida albicans that can occur in immunocompromised individuals. Classically, it manifests on physical exam with sore throat and dysphagia, friable white plaques and erythematous buccal mucosa, like those seen in the image below. This infection can also present as diaper rash in infants or as a diffuse mucocutaneous fungal infection in severely immunosuppressed individuals. Nystatin "swish and swallow" is often used to treat oral candidiasis; amphotericin B or fluconazole can be used for serious systemic infection. Nystatin kills fungi by ?

Answer 19

binding ergosterol to form pores in the surface membrane.

Griseofulvin is an antifungal agent that binds tubulin, interfering with microtubule function. It effectively treats fungal infections of the skin (such as tinea capitis, corporis, and pedis). Acyclovir, an antiviral nucleoside analog that inhibits DNA replication, can be used to treat herpes simplex virus. CMV infections are typically treated with ganciclovir, an antiviral nucleoside analog that inhibits DNA polymerase by mimicking pyrophosphate. Trimethoprim-sulfamethoxazole is considered first-line therapy for Pneumocystis pneumonia. It impairs DNA replication by inhibiting folate synthesis.

Question 20

Candida stomatitis typically presents in immunocompromised individuals. Symptoms include dysphagia, sore throat, and white friable plaques on exam. Nystatin “swish and swallow” can be used to treat oral candida infections. Nystatin kills candida by ?

Answer 20

binding ergosterol and forming pores in its membrane.

Question 21

This patient, who lives and vacations in the U.S. northeast, appears to have a facial nerve palsy secondary to Lyme disease, which is caused by Borrelia burgdorferi. The vector is the Ixodes tick, which is found commonly in the northeastern United States. In many cases, the patient doesn’t even recall being bitten by a tick, and not every case presents with the characteristic bull’s-eye rash.
Facial nerve palsy can be differentiated from?

Answer 21

stroke by asking the patient to furrow his forehead. The occipitofrontalis muscle is controlled by both motor cortices, so an inability to furrow the forehead is either the result of two strokes in the same precise location in both hemispheres (unlikely) or a result of dysfunction of the lower motor neuron of cranial nerve VII (likely).

Question 22

In addition to facial nerve palsy, Lyme disease can manifest with ?

Answer 22

transient arthralgias, atrioventricular nodal block, secondary annular rashes, and carditis. Cardiac involvement is seen in 5% of patients, whereas neurologic involvement is seen in 15%. Atrioventricular block can be symptomatic and high-grade (ie, third-degree).

Question 23

Lyme disease progresses through three phases:

1. Early localized phase: flu-like symptoms and the classic target-shaped rash, erythema chronicum migrans.
2. Early disseminated phase: neurologic involvement (facial palsy) or?

Answer 23

cardiac involvement (atrioventricular conduction delay).
3. Late Lyme disease: asymmetric arthritis or (rarely) subacute encephalopathy.
Doxycycline is the antibiotic treatment of choice for most patients. However, for those with cardiovascular or neurologic involvement, intravenous ceftriaxone is the treatment of choice.

Of note: Bell palsy is not synonymous with facial nerve palsy. Bell palsy is a facial nerve palsy of idiopathic origin. If this patient were to test negative for Lyme serology, he would be diagnosed with Bell palsy.

Question 24

A reduced ankle-brachial index is seen in peripheral arterial disease, most likely due to atherosclerotic disease. It often manifests with claudication that may occur with exercise or even at rest in more severe cases.
Patients with atrial fibrillation who are not receiving an anticoagulant are at risk of developing an atrial thrombus that may cause an embolic stroke by occlusion of an cerebral artery when dislodged. If an artery in the brain is occluded, stroke results.
Carotid stenosis is a risk factor for ?

Answer 24

the development of stroke. It can be managed medically or, if severe or symptomatic, with surgery (carotid endarterectomy). It can often be detected by hearing a bruit of the carotid artery.
Patients that have luminal narrowing of a vessel in the brain due to atherosclerosis are at risk for stroke. If the disease process is also present in the coronary vessels, myocardial ischemia can result.

Question 25

Lyme disease is caused by the spirochete Borrelia burgdorferi. This disease can move from a primary phase to a secondary phase if untreated. The secondary phase can have?

Answer 25

cardiac (AV block) or neurologic complications (eg, facial nerve palsy). The third, late, phase of Lyme disease is characterized by asymmetric arthritis or subacute encephalopathy.

Question 26

This patient has evidence of a systemic inflammatory process (elevated WBC count, low blood pressure, tachypnea, and tachycardia) with a pleural effusion and mediastinal widening—an enlarged area of opacification between the lungs—on a chest x-ray. In the context of his recent history of travel to a farm and possible contact with ruminant animals, he likely has?

Answer 26

inhalation anthrax.
Bacillus anthracis are aerobic gram-positive rods that grow in chains with a protein capsule. (See image.) These organisms can cause cutaneous, inhalation, or gastrointestinal anthrax. Spores from sheep or goat skin are the primary mode of transmission of inhalation anthrax.

Question 27

Inhalation anthrax, also known as “wool-sorter’s disease,” usually has two phases. The first phase, characterized by malaise, dry cough, and chest pressure, resolves in a few days. During the second phase, patients suddenly develop?

Answer 27

acute respiratory distress and hypoxemia, followed by hemorrhagic mediastinitis and bloody pleural effusions. Mediastinal widening is a classic radiologic finding. If a patient is not rapidly treated with penicillin, doxycycline, ciprofloxacin, or levofloxacin, systemic infection can cause septic shock (due to exotoxins produced by the bacteria) and death within 24 hours.

Question 28

B. anthracis are the only medically relevant bacteria with a protein capsule. Raxibacumab, a monoclonal antibody that targets the protective antigen of the anthrax toxin, is used to treat ?

Answer 28

inhalation anthrax after exposure to B. anthracis. It is used in combination with appropriate antimicrobial therapy and as prophylaxis of inhalation anthrax when alternative therapies are unavailable or inappropriate.

Question 29

Brucella species are described as anaerobic gram-negative pleomorphic coccobacilli that do not require cysteine for growth and are transmitted from cattle, sheep, or goats to humans; systemic symptoms include undulant fever and lymphadenopathy. Francisella tularensis are aerobic gram-negative pleomorphic coccobacilli requiring cysteine for growth and cause tularemia, characterized by?

Answer 29

abrupt onset of fever, chills, malaise, and fatigue. Nocardia asteroides are aerobic gram-positive organisms with weakly acid-fast rods forming long branching filaments and cause pulmonary infections, primarily in immunocompromised individuals.

Question 30

Legionella pneumophila cause a severe pneumonia (Legionnaires’ disease) and may be described as poorly stained gram-negative rods that are visualized best on silver stain with iron and cysteine. Several organisms that cause respiratory tract infections are aerobic gram-positive rods in chains with a polysaccharide capsule, including Streptococcus pneumoniae, Haemophilus influenzae, and Pseudomonas aeruginosa. However, none of these pathogens cause ?

Answer 30

a hemorrhagic pleural effusion. Anaerobic gram-positive rods in chains with a protein capsule, such as Clostridium and Actinomyces spp., can cause disease in humans; however, symptoms do not include respiratory tract infection.

Question 31

Inhalation anthrax is rapidly fatal and is characterized by hemorrhagic mediastinitis, mediastinal widening, and bloody pleural effusions. It is caused by?

Answer 31

Bacillus anthracis, a gram-positive rod with a protein capsule, whose spores are often transmitted from sheep or goat skin.