Micro USMLE 8-21 (4)

Question 1

The patient arrives with otitis externa, which often occurs after swimming. Otitis externa typically manifests with ear pain, pruritus, discharge, and pain on manipulation of the pinna. Periauricular cellulitis and fever are signs of more serious infection. Progression of this disease can lead to osteomyelitis of the bones of the skull and cranial nerve damage. As opposed to otitis media, with otitis externa there is typically no evidence of middle ear fluid. Pseudomonas aeruginosa is the most common cause of?

Answer 1

otitis externa (39% of cases).

Haemophilus influenzae, prior to the HiB vaccine, commonly caused otitis media and meningitis, but not otitis externa.
Moraxella catarrhalis is another common cause of otitis media and pneumonia, especially in COPD patients.
Streptococcus pyogenes, or group A streptococci, causes a litany of infections, including cellulitis, pharyngitis, scarlet fever, and impetigo. Although it is sometimes implicated in otitis media, it rarely causes otitis externa.
Streptococcus pneumoniae is the most common cause of otitis media but not otitis externa.

Question 2

Otitis externa is an acute infection of the ear canal, typically caused by bacteria, with Pseudomonas aeruginosa being the most frequent causative agent, especially after swimming. Otitis media is most commonly caused by?

Answer 2

Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis.

Question 3

This patient with pearly white/skin-colored papules with central umbilication has the classic dermatological finding associated with molluscum contagiosum. Caused by a?

Answer 3

poxvirus, an enveloped virus with double-stranded, linear DNA, molluscum contagiosum virus is most frequently seen in children. In this population, the virus is often transmitted through skin-to-skin contact or indirect contact with fomites (e.g. gymnasium equipment). Lesions commonly occur on the chest, arms, trunk, legs, face, and intertriginous areas.
Molluscum contagiosum also occurs in adults and may be transmitted sexually. In such cases, if the patient is immunocompetent, lesions tend to be located around the genitalia and surrounding areas, as seen in this patient. Widespread, persistent, and atypical presentations may be seen in immunocompromised patients. The infection often resolves spontaneously, but may require topical therapy or surgery in immunocompromised patients.

Question 4

The organisms named in the alternate answer choices do not produce the pearly white or skin-colored papules with central umbilication seen in this patient’s skin. Among the non-enveloped RNA viruses, coxsackie virus, which causes hand, foot, and mouth disease, is most closely associated with a rash; however, this is described as red, blister-like lesions. Microsporum, a mold, (and several similar organisms) cause cutaneous infections with a pruritic, flat rash on several parts of the body. Malassezia species, al-shaped, budding yeasts, are responsible for tinea versicolor, an infection in which patches of skin become discolored due to?

Answer 4

melanocyte damage by the infecting fungus. Rickettsia are gram-negative, obligate intracellular bacteria capable of causing a rash, which is erythematous; other similar organisms also cause rashes different from those in the illustration. Infections caused by this pathogen often present with systemic symptoms, as well. Dermatological findings of Treponema pallidum, a spiral-shaped bacterium, include maculopapular rashes, characterized by flat, coarse red areas of skin; other spirochetes also cause rashes.

Molluscum contagiosum manifests as white or skin-colored papules or nodules with central umbilication. It often resolves spontaneously.

Question 5

This patient just returned from an adventure trip in Hawaii, where he likely hiked the countryside and swam in the lakes. His symptoms include headache, fever, fatigue, vomiting, jaundice, conjunctival suffusion, and myalgias, notably in the calves. These symptoms, plus his recent recreational exposure to freshwater, are suggestive of leptospirosis.

Leptospirosis is caused by the spirochete Leptospira interrogans, which is found in water (especially in tropical climates) that has been contaminated by the urine of infected animals. Leptospirosis is common after recreational freshwater exposures. Patients commonly have?

Answer 5

characteristic red eyes from conjunctival suffusion with subconjunctival hemorrhage. Myalgia of the calves is a classic symptom of this infection.

This patient’s lab tests reveal the additional signs of renal failure and hepatitis. The combination of leptospirosis with renal failure and hepatitis is known as Weil disease, a more severe form of infection by Leptospira interrogans. Other complications include pulmonary hemorrhage, acute respiratory distress syndrome (ARDS), myocarditis, rhabdomyolysis, and uveitis.

Question 6

Hepatitis A and Rocky Mountain spotted fever (RMSF) do not typically present with renal failure, which is a key feature in this patient, nor is RMSF endemic to Hawaii. Lyme disease often has cutaneous manifestations and also does not present with and is also associated with arthralgia. Naegleria meningitis is associated with?

Answer 6

fresh water exposure and presents with severe meningoencephalitis, which would manifest with nuchal rigidity, altered mental status, photophobia, and/or papilledema.

Leptospirosis, which is caused by Leptospira interrogans, presents with flu-like symptoms, myalgia (classically of the calves), jaundice, and conjunctival suffusion. Weil disease is a more severe form of leptospirosis and presents with the additional symptoms of renal failure and hepatitis.

Question 7

This patient presents with watery diarrhea and abdominal pain of 3 days’ duration and has been in daycare with children who have the same signs and symptoms. This patient likely has a rotavirus infection, which is a common cause of viral gastroenteritis and fatal diarrhea in unvaccinated children between the ages of 6 months and 2 years. This child also has increased capillary refill time, dry mucous membranes, elevated serum osmolality, and tachycardia, which are signs of dehydration most likely caused by the profuse diarrhea. Rotavirus is part of the reovirus family and is ?

Answer 7

a double-stranded, nonenveloped, linear-segmented RNA virus. It is worth noting that in more developed countries, norovirus and other caliciviruses are becoming the more common causes of viral gastroenteritis due to routine vaccination against rotavirus.

Question 8

Single-stranded enveloped RNA viruses with positive polarity include togaviruses (rubella) and coronaviruses (“common cold”).
Single-stranded enveloped RNA viruses with negative polarity include rhabdovirus (eg, rabies virus), paramyxoviruses (parainfluenza or croup), and orthomyxoviruses (influenza).
Double-stranded enveloped DNA virus describes a ?

Answer 8

poxvirus (eg, smallpox or molluscum contagiosum).

Nonenveloped, double-stranded DNA virus describes an adenovirus.

Question 9

Rotavirus is a common cause of viral gastroenteritis in unvaccinated children 6 months to 2 years of age. Symptoms include watery diarrhea, abdominal pain, and dehydration. Rotavirus is a member of the reovirus family and is?

Answer 9

a double-stranded, linear-segmented RNA virus.

Question 10

This patient presents with chest pain that improves on leaning forward and worsens with inspiration, diffuse ST-segment elevations on an ECG, and a history of a viral illness. On auscultation, a pericardial friction rub is apparent, which accounts for the scratchy, leathery sound during both systole and diastole (as heard in the audio clip). Together, these findings suggest ?

Answer 10

pericarditis caused by the coxsackie B virus. Classic ECG findings include diffuse ST-segment elevation and depression of the PR segment (see image).
Pericarditis is frequently preceded by a viral upper respiratory tract infection. Although many viruses may cause pericarditis, coxsackie B is one of the most common causes of inflammation of the pericardial membrane. Coxsackievirus is a picornavirus, the smallest of the RNA viruses. They are positive, single-stranded, naked, icosahedral RNA viruses. Additionally, coxsackievirus is a common cause of viral myocarditis.

Question 11

Catalase-positive, coagulase-positive cocci describes S. aureus, which typically causes endocarditis.
Double-stranded, linear, enveloped, icosahedral DNA virus describes herpesviridae, including cytomegalovirus and Epstein-Barr virus.
Double-stranded, segmented RNA virus describes ?

Answer 11

reoviridae. Rotavirus (a member of this family) can cause diarrhea in children.
Positive, single-stranded, helical RNA virus describes the structure of coronaviridae. A coronavirus typically causes a cold-like syndrome.

Question 12

The patient’s symptoms of shortness of breath and palpitations with associated biventricular dilatation and increased PCWP suggest a cardiac pathology. The presence of intracellular protozoan parasites in a patient from Peru further suggests myocarditis caused by Trypanosoma cruzi. This infection, known as Chagas disease, is endemic in many areas of South America.
The infection is transmitted by?

Answer 12

reduviid bugs, also known as “kissing bugs.” Reservoirs of T. cruzi include wild animals, such as rodents and raccoons; dogs; and humans. T. cruzi is an intracellular protozoan that localizes mainly in the heart and nerve cells of the myenteric plexus, leading to myocarditis and dysmotility of hollow organs, such as the esophagus, colon, and ureters.

Question 13

Cardiac involvement manifests with ventricular dilatation and congestive heart failure secondary to myocyte necrosis and fibrosis. Intracellular parasites can be visualized in tissue sections, as shown in the image. Chagas disease is also a cause of acquired achalasia. This is due to loss of innervation to the lower esophageal sphincter, which prevents relaxation of the sphincter and leads to dilation of the distal third of the esophagus.

The Aedes aegypti mosquito can spread Zika virus, but it is most commonly linked to?

Answer 13

the birth defect microcephaly. The Ixodes tick commonly causes babesiosis and Lyme disease. Phlebotomus sandflies are associated with visceral and cutaneous leishmaniasis. Tsetse flies are associated with African trypanosomiasis and may cause lymphadenopathy and somnolence.

Question 14

Chagas disease, which manifests with myocarditis, achalasia, and megacolon, is caused by T. cruzi, which are intracellular protozoan parasites transmitted by ?

Answer 14

reduviid bugs.

Question 15

This baby born to a mother who did not receive prenatal care presents with erythematous conjunctiva, swollen lids, and purulent ocular discharge, consistent with acute bacterial conjunctivitis. Given the time of presentation at 13 days of life, the likely cause is infection with ?

Answer 15

Chlamydia trachomatis (serotypes D through K), which is the most common sexually transmitted disease in the United States. In addition to conjunctivitis, Chlamydia trachomatis may cause pneumonia.
Neonates are exposed to the organism during vaginal delivery. Symptoms of chlamydial (inclusion) conjunctivitis begin 5–14 days after birth, and include inflammation, swelling around the eyelids, and the presence of a yellow purulent discharge. Neisseria gonorrhoeae is another common cause of neonatal conjunctivitis; however, symptoms typically present between two and five days after birth.

Question 16

Gastroenteritis tends to be uncommon in neonates, and bacterial gastroenteritis is exceedingly uncommon in this age group. Meningitis is caused by several neonatal pathogens and infants of mothers without prenatal care are at higher risk, but Chlamydia trachomatis does not cause meningitis. Chorioretinitis is associated with congenital toxoplasmosis. Urethritis is uncommon in neonates, although urinary tract infection (cystitis, pyelonephritis) is a common cause of fever of unknown origin in pediatrics, but Chlamydia trachomatis is not a typical pathogen. Fever may be an unreliable sign of infection in neonates; but if present is associated with?

Answer 16

a high risk of severe bacterial infection.

Question 17

Neonatal chlamydial infection (serotypes D through K) produces inclusion conjunctivitis 5–14 days after birth. Another complication of neonatal chlamydial infection is ?

Answer 17

pneumonia, which occurs between 4 and 11 weeks after birth.

Question 18

This patient with vulvar pruritus, dysuria, and vaginal discharge recently received a 10-day course of antibiotics. Combined with the findings of hyphae and spores observed on Gram staining, her symptoms and medication history strongly suggest a diagnosis of vulvovaginal candidiasis. This infection is generally detected in association with?

Answer 18

a normal vaginal pH range (<4.5).

A pH of 1.0 is not generally associated with vaginal infections. A vaginal pH greater than 4.5 may be associated with the following:

Question 19

5. 0–6.0 Bacterial vaginosis
6. 0–7.0 Trichomonas vaginalis
7. 0 ?

Answer 19

Presence of cervical mucus or semen.

Vulvovaginal candidiasis is generally detected in association with a normal vaginal pH range (<4.5). A thick white discharge with no odor is often found on speculum examination.

Question 20

This patient presents with a cough with hemoptysis of several months’ duration, fatigue, weight loss, and dry skin. In the context of his history of intravenous (IV) drug use, the likely diagnosis is pulmonary tuberculosis (TB). Posttussive crackles are a common physical examination finding in individuals with TB. The patient’s history of IV drug use should also raise suspicion for untreated HIV, since individuals with this condition may not have a positive PPD response, even if they are infected, because of anergy from immunosuppression. This patient also has a negative Candida antigen test result, indicating that he is immunosuppressed; therefore a negative PPD test result should not be viewed as valid.

The severity of his symptoms and the presence of cavitary lesions suggest that he has?

Answer 20

secondary TB, which results from reactivation of a latent infection or re-exposure to the pathogen after a diagnosis of primary TB. As shown in the CT scan provided with the question, secondary TB elicits a rapid and profound tissue response, usually in the lung apex, as well as cavitation (as evidenced by the multiple lung lesions). Cavitation is not usually seen in patients with primary TB. TB is caused by Mycobacterium tuberculosis (shown in the image below). M. tuberculosis can be identified by a Ziehl-Neelsen stain, also known as the acid-fast stain, which penetrates the waxy coat of the pathogen. Acid alcohol is used to decolorize non–acid-fast cells; acid-fast cells resist this decolorization.

Question 21

Because of the patient’s social history and cavitary lung lesions with persistent fever, an infectious process is more likely than a malignancy. Chromogranin stain is used in immunohistochemical evaluation of neuroendocrine tumors. The classic presentations of Lyme disease, malaria, African sleeping sickness, Chagas disease, and Chlamydia are not seen, making Giemsa stain an incorrect choice. Even though this patient is immunocompromised, without symptoms of meningitis or encephalitis, infection with Cryptococcus neoformans is unlikely, and therefore India ink is not a correct choice. Finally, because the patient is not presenting with the classic triad of cardiac symptoms, arthralgias, and neurologic symptoms, he is unlikely to have?

Answer 21

Whipple disease, an infection with Tropheryma whipplei, which is detected by periodic acid–Schiff stain.

Question 22

The patient presents with right-sided weakness, left eye blindness, and cognitive impairment. These symptoms, combined with her MRI and her low CD4+ count, are consistent with progressive multifocal leukoencephalopathy (PML). PML results from an infection by the JC virus, which is often acquired during childhood and remains inactive until something—such as a weakened immune system caused by HIV—allows it to be reactivated. It can occur in HIV patients with CD4+ counts <200/mm3.
PML typically manifests with rapidly progressive focal neurologic deficits without signs of increased intracranial pressure. Ataxia, aphasia, and cranial nerve deficits may also occur. Lumbar puncture is nondiagnostic and frequently demonstrates mild elevations in protein and WBCs. Cerebrospinal fluid (CSF) analysis is often normal, though it can reveal the presence of myelin basic protein, which is due to ?

Answer 22

demyelination caused by the JC virus.

Typical PML imaging findings show patchy areas of low T1 signal and high T2 signal in the subcortical white matter. Lesions are often bilateral and asymmetric, although, despite the name of the condition, they may be unifocal. The lesions exert no mass effect. The image in the vignette shows nonenhancing single or multiple white matter lesions on T2-weighted hyperintensities, indicated by the arrows.

Although stereotactic biopsy is required for definitive diagnosis, a positive CSF polymerase chain reaction for JC virus DNA is diagnostic in the appropriate clinical setting. Histology of the lesions shows nuclear inclusions in oligodendrocytes. Although there is no definitive treatment for this otherwise relentlessly progressive and fatal condition, clearance of JC virus DNA and clinical improvement can be observed in response to highly active antiretroviral therapy and recovery of the CD4+ cell count.

Question 23

Cortical tuberculomas are caseating foci within the cortical parenchyma occurring from previous hematogenous mycobacterial bacillemia. It has different imaging appearance and is uncommon in the developed world.
Cytomegalovirus (CMV) encephalitis can mimic the appearance of PML, but would be more likely associated with enhancing periventricular white matter lesions in cortical and subependymal regions.
Primary central nervous system lymphoma typically affects those with CD4+ cell counts <50/mm3. MRI will demonstrate one or more enhancing lesions (50% are multiple and 50% are single) that typically are surrounded by?

Answer 23

edema and can produce a mass effect.
Space-occupying lesions due to toxoplasmosis infection represent the most common cause of cerebral mass lesions in HIV-infected patients and typically manifest with multiple enhancing lesions on MRI.

Question 24

CD4 counts <100 cells/mm3 make HIV-positive patients susceptible to a number of opportunistic infections. In particular, retinitis is associated with cytomegalovirus (CMV) infection. CMV retinitis manifests with rapidly diminishing sight and loss of central vision with floaters and blind spots. On fundoscopic exam (see image), cotton-wool exudates, necrotizing retinitis, and perivascular hemorrhages are usually seen.
Retinal detachment, in which the neurosensory layer of the retina separates from the outermost pigmented epithelium, can occur as a later finding of CMV retinitis due to ?

Answer 24

the accumulated damage to the retina. Retinal detachment typically manifests with sudden onset of floaters and/or photopsias with monocular vision loss. CMV retinitis is treated with the antivirals ganciclovir and foscarnet.

Question 25

A cherry-red spot on the macula is found in Tay-Sachs disease, Niemann-Pick disease, and central retinal artery occlusion. Microaneurysms and neovascularization can be seen in diabetic retinopathy. Papilledema is found in ?

Answer 25

situations of increased intracranial pressure.

Question 26

CMV retinitis, a common cause of rapidly progressing blindness in immunocompromised patients, manifests with cotton-wool exudates, perivascular hemorrhage, and necrotizing retinitis on fundoscopic examination. Retinal detachment is a severe complication of CMV retinitis, which can occu with sudden onset of floaters and/or photopsias with monocular vision loss. CMV retinitis is treated with ?

Answer 26

ganciclovir and foscarnet.

Question 27

This patient presents with bloody diarrhea after eating food from street vendors while on vacation. In addition, his pale complexion and fatigue suggest either anemia or dehydration. These symptoms increase the suspicion for the enterohemorrhagic strain of Escherichia coli (EHEC).
EHEC infection usually results in self-limiting, bloody diarrhea lasting 5–10 days, but may cause hemolytic uremic syndrome (HUS). This is seen most commonly in young and elderly patients infected with the O157:H7 serotype of E. coli. This bacterium gains its potential to cause ?

Answer 27

HUS by picking up the Shiga-like toxin via specialized (lysogenic) transduction.

HUS is characterized by the triad of anemia, thrombocytopenia, and acute renal failure. HUS complicates up to 9% of EHEC infections and usually begins 5–10 days after the onset of diarrhea. HUS is the most common cause of acute renal failure in children in the United States, and about half of patients will require dialysis. Thus, the most important tests to rule out renal failure are renal function tests.

Question 28

Common sources of infection by the O157:H7 serotype of E. coli include swimming pools, direct contact with people and animals, ingestion of food, and drinking water. Contaminated ground beef is the most common cause of food-borne outbreaks. Produce-associated outbreaks, however, are also common.

In some cases, HUS can be associated with ?

Answer 28

hepatomegaly and elevated liver enzymes, but this would not have any serious consequences or require special treatment. Cardiac manifestations indicated by cardiac enzymes have been recorded, but are very rare, and would only be investigated if the patient was symptomatic. No useful information would be obtained from urine culture or pulmonary function tests.

Question 29

E. coli is a potential cause of gastrointestinal disease The EHEC strain can be responsible for bloody diarrhea. When it obtains Shiga-like toxin from a lysogenic phage, it can cause ?

Answer 29

hemolytic-uremic syndrome, characterized by hemolytic anemia, thrombocytopenia, and renal failure.