Infection Flashcards
- A 45-year-old man presents to emergency
department with a history of fever and rigors
for 2 weeks. On examination he is pyrexial
tachycardic and noted to have splinter hemorrhages on right middle and index fingers.
Where on the echocardiogram are vegetations most likely to occur?
a. Aortic and mitral valve
b. Aortic valve
c. Mitral valve
d. Pulmonary valve
e. Tricuspid valve
**c—Mitral valve
**
Cryptococcosis is usually acquired through the
Valve involvement in order of decreasing frequency is mitral>aortic>aortic and mitral>tricuspid>pulmonary (rare). Vegetations occur on
low pressure surface of the valve, that is, atrial
aspect of mitral, ventricular aspect of aortic. About
70% of cases of IE on native valves are caused by
viridans group Streptococci (usually after dental
infection), Streptococcus bovis and Enterococcus.
Another 25% are caused by Staphylococcus infections and these are more acute and severe.
- A 61-year-old man presented with a right frontal cerebral abscess which was drained under
image-guidance, and investigations revealed
infective endocarditis (IE). Blood cultures
and pus samples grewStreptococcus bovis.Which
one of the following is the next appropriate
investigation?
a. Bronchoscopy
b. Chest CT
c. Colonoscopy
d. Repeat blood cultures
e. Transesophageal echo
c—Colonoscopy
This patient has presented with probable meningitis, but with evidence o
S. bovis bacteremia is associated with colorectal
cancer in a significant number of cases.
A 72-year-old woman presents 3 months after
having a metallic mitral valve replacement.
She was fatigued, pyrexia 38.6 °C (101.4 °F),
BP 155/80, and pulse rate 124. On auscultation she had a mechanical first heart sound
and a late diastolic murmur best heard at the
apex. Blood cultures were taken from three
sites. What is the most likely organism?
a. Candida
b. Coagulase negative Staphylococcus
c. Gram negative bacilli
d. No growth
e. Staphylococcus aureus
e—Staphylococcus aureus
Primary amoebic meningoencephalitis (PAM) is
Prosthetic valves are involved in 10-20% of cases
of IE and eventually 5% of all prosthetic valves
will become infected. Early prosthetic valve IE
(<60 days postop) is associated with coagulase
negative staph, Candida, and gram negative
bacilli. Late prosthetic valve infection (>60 days
postop) is associated with S. aureus, V. streptococci,
and enterococci. Metallic valves more likely to
become infected <3 months while bioprosthetic
valves at >1 year.
- Which one of the following antibiotics are
associated with high risk of Clostridum difficile
pseudomembranous colitis?
a. Clindamycin
b. Gentamicin
c. Metronidazole
d. Tetracycline
e. Vancomycin
a—Clindamycin
- Central line infections are most commonly
caused by which one of the following?
a. Candida
b. Enterococci
c. Gram negative bacilli
d. Staphylococci
e. Streptococci
d—Staphylococci
Intravascular catheter-related infections are a
major cause of morbidity and mortality in the
intensive care unit. Risk factors: host predisposing factors include immunosuppression, burns,
malnutrition, use of total parenteral nutrition, and extremes of age; however these are not modifiable. The risk of infection increases after day 3.
Higher rates of infection in adults are seen with
femoral vein insertion sites, then jugular and
the least with subclavian. The skin pathogens
involved are Staphylococci (coagulase-negative
and S. aureus; 50%), Gram negative bacilli (30%),
enterococci (10%), and Candida species (10%).
Paired blood cultures can be obtained from a
peripheral vein and via the catheter [bearing in
mind that false-positives (i.e. contaminants) are
more common from line cultures]. Culture of a
catheter tip is only useful if the catheter is thought
to be infected. If line related sepsis is suspected, the
catheter should be removed, cultures taken
(peripheral, tip of central venous catheter) and antimicrobial therapy should beinitiated.Management
is guided by the identification of the causative
organism: S. aureus bacteremia (SAB) should be
treated for at least 14 days, coagulase negative
Staphylococcus for 5-7 days, Gram negatives for
10-14 days, and Candida for 14 days. Echocardiography is absolutely indicated in SAB. Most centers
have rates of MRSA high enough to empirically
include cover for this pathogen until culture results
are available. Prevention is via the use of strict aseptic technique by an appropriately experienced person, hand washing/strict aseptic technique, full
barrier precautions, chlorhexidine for skin disinfection, antiseptic impregnated sponge dressings. Vigilant catheter care is essential, in particular
antiseptic wipes on ports before access. All catheter
insertion sites should be assessed daily for infection.
Removal of unnecessary catheters is vital, but often
overlooked. If inserted under emergency conditions they should be removed/replaced within
48 h. Guide-wire exchange techniques should not
be used, as higher rates of bacteremia result.
- Which one of the following statements
regarding ventilator associated pneumonia
(VAP) in adults is most accurate?
a. Defined as pneumonia that occurs more
than 1 week after endotracheal intubation
b. Risk factors include age over 70, aspiration and depressed conscious level
c. Methicillin-resistant Staphylococcus aureus
(MRSA) is responsible in >50%
d. Diagnosis is primarily made on chest Xray showing bilateral infiltrates
e. Antibiotics should not be started until
microbiological confirmation
a—Defined as pneumonia that occurs more
than 1 week after endotracheal intubation
Hospital-acquired pneumonia (i.e. pneumonia
that begins 48 h or more after admission) is the
leading cause of hospital acquired infection leading to mortality. VAP occurs 48 h or more after
endotracheal intubation.
Mechanical ventilation
increases the risk of pneumonia due to micro aspiration of oropharyngeal microorganisms via
leakage around the endotracheal tube cuff or
directly through the tube. Risk factors for VAP
include: age over 70 years, chronic lung disease, depressed consciousness, and aspiration. The key
modifiable factors increasing risk for VAP are previous antibiotic exposure, use of paralytic agents,
re-intubation or prolonged intubation, frequent
ventilator circuit changes, presence of a nasogastric tube, or intra-cranial pressure monitor.
Local epidemiology varies significantly, but data
from a large US study showed the major pathogens to be MRSA (14.8%), Pseudomonas aeruginosa
(14.3%), and other Staphylococcus species (8.8%).
A combination of clinical, microbiological, and
radiological criteria is required for diagnosis.
The principles of treating VAP include early antimicrobial therapy after appropriate specimens are
taken (guided by the local microbiology), then deescalation according to culture and susceptibility
results. In many units this will result in empiric
combination therapy to cover multi-resistant
organisms. A total duration of 5-7 days of effective
therapy is adequate for most pathogens, though
most physicians would treat longer for Pseudomonas or true staphylococcal pneumonia. Prevention
strategies include reducing need for ventilation,
reducing colonization, and reducing aspiration.
- Which one of the following is not a mechanism of antibiotic resistance?
a. Decreased permeability of cell to antibiotic
b. Efflux pump (active transport)
c. Inactivating enzyme
d. Sequestration in cytoplasmic vesicles
e. Target site modification
d—Sequestration in cytoplasmic vesicles
- Which one of the following antimicrobials is
not first line therapy for active Mycobacterium
tuberculosis infection?
a. Ethambutol
b. Isoniazid
c. Pyrazinamide
d. Rifampicin
e. Streptomycin
e. Streptomycin
M. tuberculosis is transmitted by respiratory secretions (e.g. coughing). It does not stain well with
the Gram stain (cell wall contains mycolic acids
rather than peptidoglycan), but is visualized with
the acid-fast Ziehl-Neelsen stain and grows on
Lowenstein-Jensen agar. Several characteristics
of the tubercle bacillus make it difficult to control
quickly. One problem is its intracellular location,
where drugs do not penetrate well. In addition,
the bacillus is often found in large cavities with
avascular centers, into which drugs do not penetrate well either. Finally, the tubercle bacillus has
a very slow generation time. First line therapy
includes: rifampin, isoniazid (also used for prophylaxis), pyrazinamide and ethambutol (RIPE
therapy). They are used in combination because
there is a high incidence of resistance to these
drugs. Side effects are outlined here. Isoniazid:
peripheral neuropathy (relative pyridoxine deficiency), hepatotoxicity, lupus-like syndrome,
P450 inhibitor. Rifampin induces hepatic P-450
enzymes, including those that metabolize opioids,
as turning urine, sweat, tears an orange color.
Ethambutol has a side effect of optic neuropathy
(decreased visual acuity and color blindness).
- Which one of the following statements about
human immunodeficiency virus (HIV) infection is most accurate?
a. Pneumocystis carnii pneumonia is likely if
the CD4 count is 300
b. Toxoplasma encephalitis is likely when
the CD4 count is 200
c. In an HIV positive individual AIDS is
diagnosed when CD4 count is below 200
d. HIV seroconversion usually occurs 2-6
weeks after infection without a drop in
CD4 count
e. Highly active antiretroviral therapy is
normally initiated when the CD4 count
is below 400
c—In an HIV positive individual, AIDS is
diagnosed when CD4 count is below 200
HIV is an encapsulated retrovirus. Two viral
envelope glycoproteins, gp120 and gp41, allow
the virus to infect CD4+ T cells (as well as some
macrophages, dendritic cells, and microglial cells)
that express an appropriate coreceptor (the chemokine receptors CXCR4 or CCR5). Upon entry
into the cell, the virus efficiently copies its RNA
genome into double-stranded DNA using the
viral enzyme reverse transcriptase. The viral
DNA copy is integrated into the host cell
genome, aided by the viral enzyme integrase.
This pro-virus form may remain latent in the cell
until its expression is signaled. Expression of
functional proteins by the virus involves a virally
encoded protease that cleaves polyproteins into
smaller functional proteins. When the provirus
is expressed to form new virions, the host cells will
often lyse. With acute HIV infection, the individual
may remain asymptomatic or develop an acute
illness that resembles influenza or infectious
mononucleosis; symptoms usually develop within
2-6 weeks after infection. During this stage,
antibodies to HIV are generally undetectable.
Seroconversion usually occurs during clinical
latency, an asymptomatic period that would last
approximately 7-10 years in an untreated patient.
Low-level (but persistent) replication of HIV
causes a gradual decrease in CD4+ T cells, and minor opportunistic infections may occur. During
the crisis phase, escalation of viral replication leads
to a more rapid T-cell decline. This is clinically
apparent as weight loss, fever, fatigue, and lymphadenopathy. Acquired immunodeficiency syndrome (AIDS) is the diagnosis for a person who
is HIV-positive and has a T-cell count below
200 μL1 or presents with one of the AIDSdefining opportunistic infections/malignancies
(e.g. Kaposi’s sarcoma, non-Hodgkin’s lymphoma, cervical cancer). Progression or response
to antiretrovirals (reverse transcriptase inhibitors,
protease inhibitors) is monitoredCD4+T-cell count
indicates the damage that has occurred to the
immune system (normally 500-1500 μL1
), while
viral load serves as amarker for disease progression
and drug therapy effectiveness by measuring the
amount of actively dividing HIV virus.
An 82-year-old is brought into the emergency department with pyrexia and reduced
level of consciousness. She has known
chronic obstructive airways disease and was
recently started on antibiotics by her doctors
for a lower respiratory tract infection. Her
temperature is 39 °C (102.2 °F), heart rate
124, and respiratory rate 32. Blood pressure
on arrival was 95/67. Which one of the following best classifies her condition?
a. Multiple organ dysfunction syndrome
(MODS)
b. Sepsis
c. Septic shock
d. Severe sepsis
e. Systemic inflammatory response syndrome
b—Sepsis
