A 24-year-old man has had a painful sore throat on one side for 5 days, with foul breath and bleeding gums. A grey slough covers a deep ulcer on the left tonsil. He is fully immunized against diphtheria, and a rapid group A streptococcal antigen test is negative. A Gram stain of a swab from the ulcer base shows many neutrophils, many slender fusiform gram-negative rods with pointed ends and many thin spiral organisms. The throat culture on sheep blood agar grows only mixed oral flora. The most likely diagnosis is:

Vincent angina (fusospirochetal)

Group A streptococcal tonsillitis

Throat swabs for group A streptococci are often plated on sheep blood agar containing trimethoprim-sulfamethoxazole. Why are the antibiotics added?

Suppress normal throat flora so beta-hemolysis stands out

Detect strains resistant to trimethoprim-sulfamethoxazole

Make the zones of beta-hemolysis larger

Stop Streptococcus pyogenes from overgrowing the plate

Sputum cultures for Legionella from a ventilated patient are repeatedly overgrown by mixed flora on plain BCYE agar. Which processing change is most likely to recover Legionella?

Brief acid treatment, then selective BCYE agar

Plating the sputum on MacConkey agar instead

Incubating BCYE agar anaerobically for 48 hours

Holding the sputum cold overnight before plating

A 52-year-old man with diabetes has pneumonia and a liver abscess. Sputum and blood grow Klebsiella pneumoniae. When a colony is touched with a loop and lifted, it forms a viscous string longer than 5 mm. This finding indicates:

A hypermucoviscous, often hypervirulent strain

Contamination with Klebsiella oxytoca

A carbapenemase-producing strain

A strain that has lost its capsule

Respiratory tract cultures

14 DOH Abu Dhabi (HAAD) lab licensing practice questions on Respiratory tract cultures, part of Microbiology. The first 14 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
A 24-year-old man has had a painful sore throat on one side for 5 days, with foul breath and bleeding gums. A grey slough covers a deep ulcer on the left tonsil. He is fully immunized against diphtheria, and a rapid group A streptococcal antigen test is negative. A Gram stain of a swab from the ulcer base shows many neutrophils, many slender fusiform gram-negative rods with pointed ends and many thin spiral organisms. The throat culture on sheep blood agar grows only mixed oral flora. The most likely diagnosis is:
1. AVincent angina (fusospirochetal)
2. BOropharyngeal candidiasis
3. CGroup A streptococcal tonsillitis
4. DPharyngeal diphtheria
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Answer: A · Vincent angina (fusospirochetal)
One-sided ulcerating tonsillitis with foul breath in a young adult, and a Gram stain full of fusiform gram-negative rods and spirochetes among neutrophils, is Vincent angina. The organisms are oral anaerobes, mainly Fusobacterium species with oral treponemes, so routine throat culture shows only mixed flora and does not help. The diagnosis rests on the Gram stain of the lesion, which the laboratory reports descriptively. The negative antigen test and the absence of gram-positive cocci in chains make group A streptococcal tonsillitis unlikely. In an unimmunized patient with a membrane, the laboratory would also culture on tellurite medium for Corynebacterium diphtheriae.
- B: Candida would show budding yeasts and pseudohyphae, not fusiform rods and spirochetes.
- C: Group A streptococci are gram-positive cocci in chains, and the antigen test is negative.
- D: Diphtheria shows irregular gram-positive rods, and he is fully immunized.
Question 2 · Medium
Throat swabs for group A streptococci are often plated on sheep blood agar containing trimethoprim-sulfamethoxazole. Why are the antibiotics added?
1. ADetect strains resistant to trimethoprim-sulfamethoxazole
2. BMake the zones of beta-hemolysis larger
3. CSuppress normal throat flora so beta-hemolysis stands out
4. DStop Streptococcus pyogenes from overgrowing the plate
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Answer: C · Suppress normal throat flora so beta-hemolysis stands out
Trimethoprim-sulfamethoxazole inhibits much of the normal oropharyngeal flora, while group A (and group B) streptococci still grow well on this medium. Fewer competing colonies make small beta-hemolytic colonies easier to see and pick. Some group C and G streptococci are partly inhibited, so the medium suits group A screening best. Plates are read for beta-hemolytic colonies at 24 and 48 hours.
- A: It is a selective screening medium, not a susceptibility test.
- B: The antibiotics do not enlarge hemolysis; stabbing the agar or anaerobic incubation helps show it.
- D: The medium is designed so that S. pyogenes grows while the flora is reduced.
Question 3 · Easy
Which cells in a sputum smear show that the material came from the lower respiratory tract?
1. ABudding yeast cells
2. BSquamous epithelial cells
3. CRed blood cells
4. DAlveolar macrophages
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Answer: D · Alveolar macrophages
Alveolar macrophages live in the alveoli, so finding them, along with ciliated columnar cells, shows that the specimen includes deep respiratory material. Squamous cells come from the mouth and point to saliva. Red cells and yeasts can come from the mouth or any part of the airway, so they do not show the source. Many white cells with few squamous cells are the usual mark of a good sputum.
- A: Yeasts are common in the mouth and do not mark a lower airway sample.
- B: Squamous cells line the mouth and show saliva contamination.
- C: Blood can come from the mouth, throat or lungs.
Question 4 · Hard
Sputum cultures for Legionella from a ventilated patient are repeatedly overgrown by mixed flora on plain BCYE agar. Which processing change is most likely to recover Legionella?
1. APlating the sputum on MacConkey agar instead
2. BIncubating BCYE agar anaerobically for 48 hours
3. CHolding the sputum cold overnight before plating
4. DBrief acid treatment, then selective BCYE agar
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Answer: D · Brief acid treatment, then selective BCYE agar
Legionella survives brief exposure to an acid buffer (pH about 2.2) better than most respiratory flora, so a short acid treatment reduces competing bacteria. The treated specimen is plated on BCYE agar plus a selective BCYE containing antimicrobials such as polymyxin, anisomycin and vancomycin. Plates are incubated aerobically at 35 °C in a humid atmosphere and held for at least 5 days. Legionella needs L-cysteine and iron and does not grow on MacConkey or ordinary blood agar.
- A: Legionella needs L-cysteine and iron and does not grow on MacConkey agar.
- B: Legionella is aerobic and will not grow anaerobically.
- C: Holding the specimen cold does not reduce the competing flora.
Question 5 · Hard
A 52-year-old man with diabetes has pneumonia and a liver abscess. Sputum and blood grow Klebsiella pneumoniae. When a colony is touched with a loop and lifted, it forms a viscous string longer than 5 mm. This finding indicates:
1. AContamination with Klebsiella oxytoca
2. BA hypermucoviscous, often hypervirulent strain
3. CA carbapenemase-producing strain
4. DA strain that has lost its capsule
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Answer: B · A hypermucoviscous, often hypervirulent strain
A positive string test (a string over 5 mm) marks hypermucoviscous K. pneumoniae, which is closely linked to hypervirulent strains. These strains cause community-acquired liver abscess, pneumonia, endophthalmitis and meningitis, often in people with diabetes, and were first described in East Asia. The string test says nothing about antibiotic resistance; carbapenemases are tested separately, and strains with both traits are a growing concern. Clinicians should be alerted, as they may need to look for spread to other sites.
- A: K. oxytoca is a different species, separated by indole and MALDI-TOF.
- C: Carbapenemase production is shown by phenotypic or molecular tests, not by the string test.
- D: The string reflects extra mucoid capsular material, not loss of the capsule.
Question 6 · Easy
Many laboratories accept an expectorated sputum for routine bacterial culture when the low-power (x10 objective) Gram stain fields show:
1. AFewer than 10 squamous epithelial cells
2. BMore than 25 squamous epithelial cells
3. CFewer than 10 white blood cells
4. DOnly one bacterial morphotype
Answer and explanation in LearnFree account
Question 7 · Hard
A laboratory grades sputum with these points per low-power field: neutrophils 10-25 = +1, more than 25 = +2; mucus present = +1; squamous cells 10-25 = -1, more than 25 = -2. A total of 0 or less means rejection. A sputum shows more than 25 neutrophils, mucus and 10-25 squamous cells. What is the score and decision?
1. A+1; accept for culture
2. B0; reject and request a new one
3. C+2; accept for culture
4. D+3; accept for culture
Answer and explanation in LearnFree account
Question 8 · Medium
A good-quality sputum from a patient on antibiotics grows heavy Candida albicans along with normal oral flora. The yeast most likely represents:
1. AA contaminant from the laboratory air
2. BInvasive candidiasis spread from the blood
3. CColonization of the mouth and throat
4. DCandida pneumonia needing antifungals
Answer and explanation in LearnFree account
Question 9 · Medium
For which request is the squamous cell screen, used to reject saliva-rich sputum for routine culture, not applied?
1. AHaemophilus influenzae culture
2. BStaphylococcus aureus culture
3. CRoutine bacterial culture
4. DMycobacterial culture
Answer and explanation in LearnFree account
Question 10 · Hard
A nasopharyngeal swab from an unvaccinated infant with paroxysmal cough is plated on Regan-Lowe agar. After 4 days, small shiny colonies of gram-negative coccobacilli appear. They are oxidase positive, urease negative and nonmotile, and they do not grow on MacConkey agar. The organism is most likely:
1. ABordetella bronchiseptica
2. BBordetella parapertussis
3. CHaemophilus influenzae
4. DBordetella pertussis
Answer and explanation in LearnFree account
Question 11 · Medium
A throat culture plate shows no beta-hemolytic colonies after 24 hours of incubation. What should be done next?
1. ASubculture all colonies to a new plate
2. BReport no group A streptococci at once
3. CReincubate and read it again at 48 hours
4. DRepeat the incubation at room temperature
Answer and explanation in LearnFree account
Question 12 · Medium
A throat swab from a 9-year-old with sore throat grows Staphylococcus aureus, Haemophilus influenzae and alpha-hemolytic streptococci, with no beta-hemolytic streptococci at 48 hours. The report should state:
1. ANo group A streptococci isolated
2. BStaphylococcus aureus isolated
3. CHaemophilus influenzae isolated
4. DMixed growth; please repeat the swab
Answer and explanation in LearnFree account
Question 13 · Easy
For group A streptococcal culture, a throat swab should be rubbed firmly over:
1. AThe nostrils and the lips
2. BThe tongue and the inside of the cheeks
3. CThe front teeth and the gums
4. DBoth tonsils and the back of the pharynx
Answer and explanation in LearnFree account
Question 14 · Hard
Sputum from a 14-year-old with cystic fibrosis on long-term trimethoprim-sulfamethoxazole shows tiny, non-pigmented, barely hemolytic colonies that appear only at 48 hours. They are gram-positive cocci in clusters, catalase positive, and the tube coagulase test turns positive only after 18 hours. The most likely organism is:
1. AStaphylococcus lugdunensis
2. BStaphylococcus aureus small-colony variants
3. CCoagulase-negative staphylococci
4. DMicrococcus luteus
Answer and explanation in LearnFree account

Respiratory tract cultures

Throat swabs for group A streptococci are often plated on sheep blood agar containing trimethoprim-sulfamethoxazole. Why are the antibiotics added?

Which cells in a sputum smear show that the material came from the lower respiratory tract?

Sputum cultures for Legionella from a ventilated patient are repeatedly overgrown by mixed flora on plain BCYE agar. Which processing change is most likely to recover Legionella?

A 52-year-old man with diabetes has pneumonia and a liver abscess. Sputum and blood grow Klebsiella pneumoniae. When a colony is touched with a loop and lifted, it forms a viscous string longer than 5 mm. This finding indicates:

Many laboratories accept an expectorated sputum for routine bacterial culture when the low-power (x10 objective) Gram stain fields show:

A good-quality sputum from a patient on antibiotics grows heavy Candida albicans along with normal oral flora. The yeast most likely represents:

For which request is the squamous cell screen, used to reject saliva-rich sputum for routine culture, not applied?

A throat culture plate shows no beta-hemolytic colonies after 24 hours of incubation. What should be done next?

A throat swab from a 9-year-old with sore throat grows Staphylococcus aureus, Haemophilus influenzae and alpha-hemolytic streptococci, with no beta-hemolytic streptococci at 48 hours. The report should state:

For group A streptococcal culture, a throat swab should be rubbed firmly over: