Download Cases I

Case 1
Your patient is a 10-year old girl who is receiving chemotherapy through an
indwelling venous catheter. She now has a fever of 390 C but is otherwise asymptomatic.
You do a blood culture, and the laboratory reports growth of Staphylococcus epidermidis.
Which of the following is least likely to be found by the clinical laboratory?
A.Gram-positive cocci in clusters were seen on Gram stain of the blood culture.
B.Subculture of the blood culture onto blood agar revealed nonhemolytic
colonies.
C.A coagulase test on the colonies was negative.
D.A catalase test on the colonies was negative.
Case 2
Your patient is a six-year old boy with papular and pustular skin lesions on his
face. A serous, honey-colored fluid exudes from the lesions. You suspect impetigo. A
Gram stain of the pus reveals many neutrophils and Gram-positive cocci in chains. If you
cultured the pus on blood agar, which one of the following would you be most likely to
see?
A.Small beta-hemolytic colonies containing bacteria that are bacitracin sensitive.
B.Small alpha-hemolytic colonies containing bacteria that are resistant to optochin.
C.Large nonhemolytic colonies containing bacteria that are oxidase positive.
D.Small nonhemolytic colonies containing bacteria that grow in 6.5% NaCl.
Case 3
Your patient is a 20-year-old woman with a sudden onset of fever to 1040 F
and a severe headache. Physical examination reveals nuchal rigidity. You
suspect meningitis and do a spinal tap. Gram stain of the spinal fluid reveals
many neutrophils and many Gram-negative diplococci. Of the following bacteria,
which is the most likely to be the cause?
A．Streptococcus agalactiae
B．Neisseria meningitides
C．Streptococcus pneumoniae
D．E. coli
Additional history reveals that she has had several serious infections with
this organism previously. On the basis of this, which one of the following is the
most likely predisposing factor?
A．She is HIV antibody positive
B．She is deficient in CD8-positive T cells
C．She is deficient in one of the late-acting complement components
D．She is deficient in antigen presentation by her macrophages
Case 4
Your patient is a 45-year-old woman who was part of a tour visiting the
Grand Canyon. The day before coming home, several members of the group
developed fever, abdominal cramps, and bloody diarrhea. Of the following,
which is the least likely organism to cause this infection?
A．Shigella dysenteriae
B．Salmonella enteritidis
C．Staphylococcus aureus
D．Yersinia enterocolitica
A stool culture reveals lactose-negative colonies on EMB agar and mannitol
fermenting colonies on CIN plates. Which of the following is the most likely
organism to cause this disease?
A．Shigella dysenteriae
B．Salmonella enteritidis
C．Staphylococcus aureus
D．Yersinia enterocolitica
Case 5
Your patient is a 3-year-old male who. On the day of hospital admission,
Complained of a headache and developed a fever with lethargy and emesis. He had a
two-day history of a mild upper respiratory illness. On admission, he had a fever of 39.70
C and was extremely lethargic, responsive only to deep pain. A lumbar puncture
revealed 21,000 WBC/mm3 with 90% PMNs, a CSF glucose level of 9mg/dl
(normal=40-80 mg/dl), and a CSF protein level of 1,322 mg/dl (normal=15-45 mg/dl).
The Gram stain of CSF was positive for numerous white blood cells and Gram-negative
coccobacilli. Of the following bacteria, which is the most likely to be the cause?
A．Neisseria meningitidis
B．E. coli
C．Haemophilus influenzae
D．Streptococcus agalactiae
Why are children between the ages of two months and three years particularly
susceptible to infection caused by this organism?
Case 6
Your patient is a six-year-old boy with a history of several episodes of
pneumonia. A sweat test has revealed an increased amount of chloride, indicating that
he has cystic fibrosis. He now has a fever and is coughing up thick, greenish sputum. A
Gram stain of the sputum reveals Gram-negative rods. Of the following, which one is the
most likely organism to cause this infection?
A.Pseudomonas aeruginosa
B.Haemophilus influenzae
C.Bordetella pertusis
D.Klebsiella pneumoniae
Case 7
Your patient is a 10-year-old boy who is receiving chemotherapy for acute leukemia.
He develops fever, headache, and a stiff neck, and you make a presumptive diagnosis of
meningitis and do a lumbar puncture. A Gram stain reveals a small gram-positive rod and
culture of the spinal fluid grows a beta-hemolytic colony on blood agar. Regarding this
organism, which of the following is most accurate?
A. It is non-motile
B. It produces an exotoxin that inhibits elongation factor 2
C. It is CAMP positive and bile esculin positive
D. There is a toxoid vaccine available against this organism
Case8
Your patient is a 20-year-old man who was in a fistfight and suffered a broken jaw
and lost two teeth. Several weeks later, he developed an abscess at the site of the trauma
that drained to the surface of the skin, and yellowish granules were seen in the pus.
Regarding this disease, which one of the following is most accurate?
A．The causative organism is a gram-positive rod that forms long filaments
B．The causative organism is a comma-shaped Gram-negative rod that produced an
exotoxin that increases cyclic AMP
C．The causative organism cannot be seen in the Gram stain, but can be seen in an
acid fast stain
D．A combination of gram-negative cocci and bacilli cause this disease
Case9
Your patient is a 30-year-old woman with facial nerve paralysis. She also has fever
and headache but does not have a stiff neck. On physical examination, she has a circular,
erythematous, macular rash on the back of her thigh. You suspect that she has Lyme
disease. Of the following tests, which one is the most appropriate test to order to confirm a
diagnosis of Lyme disease?
A. Blood culture to grow the organism
B. Stain for inclusion bodies within cells involved in the rash
C. Test for serum antibody against the organism
D. Dark-field microscopy
Case10
Your patient is a 60-year-old man with confusion for 2 months. He has no history of
fever or stiff neck. On physical examination, he was ataxic and his coordination was
abnormal. A diagnosis of tertiary syphilis was made based on laboratory results. Of the
following tests, which one is the most appropriate test to use to make a diagnosis of tertiary
syphilis?
A. Spinal fluid to grow the organism
B. Stain for inclusion bodies in the lymphocytes in the spinal fluid
C. Test for antibody in the spinal fluid that reacts with cardiolipin
D. ELISA for the antigen in the spinal fluid
Case11
Your patient is a 70 year-old man with a fever of 400 C and very painful
cellulites of the right buttock. The skin appears necrotic, and there are several
fluid filled bullae. crepitus can be felt, indicating gas in the tissues (In other
words, the patient has symptoms of gas gangrene). A Gram stain of the
exudates reveals large Gram-positive rods. Of the following, which one is the
most likely cause?
A．Bacillus anthracis
B．Clostridium perfringens
C．Corynebacteriun diphtheriae
D．Actinomyces israelii
Case12
A 21-year-old college student complained of malaise, low-grade fever, and
a dry, harsh cough, but not of muscle aches or pains. An x-ray revealed a diffuse
interstitial pneumonia in the left lobes of the lung. The WBC count was normal.
The student has been ill for about 1 week. Based on the information given, the
most likely organism to be causing the infection is:
A．Mycoplasma pneumonia
B．Streptococcus pneumoniae
C．Staphylococcus aureus
D．Legionella pneomophilia
Based on the information given, which of the following laboratory tests
would most rapidly assist you in making the diagnosis?
A．Cold agglutinins
B．Complement fixation test
C．Gram’s stain of sputum
D．Culture of specimen