A 5-week-old girl has had paroxysmal cough for 10 days, with episodes of apnea and cyanosis but no fever. Her mother has been coughing for a month. WBC 34.0 x 10^3/µL [34.0 x 10^9/L] with 76% lymphocytes. A nasopharyngeal specimen is collected with a calcium alginate-tipped swab, placed in a dry sterile tube, refrigerated and tested within 24 hours by a Bordetella pertussis PCR. The result is invalid because the internal amplification control fails, and repeat testing of the same specimen gives the same result. What is the most likely cause?

Inhibitors from the calcium alginate swab

Refrigeration of the specimen in transit

Interference from recent pertussis vaccine

Too few bacilli this late in the illness

After Corynebacterium diphtheriae is isolated from a throat swab, the strain's ability to cause diphtheria is shown by:

The Elek test for toxin production

A throat swab for suspected diphtheria is plated on blood agar and on:

Cystine-tellurite blood agar or Tinsdale agar

Buffered charcoal yeast extract agar

In a child with a sore throat, a negative rapid antigen test for group A streptococci should be:

Backed up with a throat culture

Accepted as ruling out infection

Upper respiratory tract

11 ASCP MLS practice questions on Upper respiratory tract, part of Microbiology. The first 11 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 5-week-old girl has had paroxysmal cough for 10 days, with episodes of apnea and cyanosis but no fever. Her mother has been coughing for a month. WBC 34.0 x 10^3/µL [34.0 x 10^9/L] with 76% lymphocytes. A nasopharyngeal specimen is collected with a calcium alginate-tipped swab, placed in a dry sterile tube, refrigerated and tested within 24 hours by a Bordetella pertussis PCR. The result is invalid because the internal amplification control fails, and repeat testing of the same specimen gives the same result. What is the most likely cause?
1. AInhibitors from the calcium alginate swab
2. BRefrigeration of the specimen in transit
3. CInterference from recent pertussis vaccine
4. DToo few bacilli this late in the illness
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Answer: A · Inhibitors from the calcium alginate swab
An invalid result with a failed internal amplification control means that something in the specimen blocked the PCR. Calcium alginate swabs release substances that inhibit PCR, so Dacron (polyester), rayon or nylon flocked swabs are used for Bordetella PCR. The infant's paroxysms, apnea and marked lymphocytosis, with a coughing household contact, strongly suggest pertussis. At 10 days of cough the bacterial DNA load is high, refrigeration is acceptable for PCR, and a 5-week-old has not yet had the first DTaP dose, usually given at 2 months. A new nasopharyngeal swab with an acceptable tip should be collected at once.
- B: Refrigeration is acceptable for PCR specimens and does not inhibit amplification.
- C: A 5-week-old has not yet received DTaP, which is usually first given at 2 months.
- D: Ten days into the cough the load is high, and a low load would not make the internal control fail.
Question 2 · Medium
After Corynebacterium diphtheriae is isolated from a throat swab, the strain's ability to cause diphtheria is shown by:
1. AThe pyrazinamidase test
2. BA metachromatic granule stain
3. CThe reverse CAMP test
4. DThe Elek test for toxin production
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Answer: D · The Elek test for toxin production
Only strains carrying the tox gene, delivered by a lysogenic bacteriophage, make diphtheria toxin and cause classic diphtheria. The Elek test is an immunodiffusion test in which toxin from the isolate forms a precipitin line with antitoxin from a filter paper strip. PCR for the tox gene is a rapid screen, but the gene can be present without toxin being made, so the Elek test confirms production. Treatment with antitoxin should not wait for these results.
- A: Pyrazinamidase helps identify C. diphtheriae (negative) but says nothing about toxin.
- B: Metachromatic granules occur in both toxigenic and nontoxigenic strains.
- C: The reverse CAMP test helps identify Arcanobacterium haemolyticum.
Question 3 · Easy
A throat swab for suspected diphtheria is plated on blood agar and on:
1. AModified Thayer-Martin agar
2. BRegan-Lowe charcoal agar
3. CBuffered charcoal yeast extract agar
4. DCystine-tellurite blood agar or Tinsdale agar
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Answer: D · Cystine-tellurite blood agar or Tinsdale agar
Tellurite media inhibit much of the throat flora, and corynebacteria reduce tellurite to form gray to black colonies. On Tinsdale agar, Corynebacterium diphtheriae also produces a brown halo from cystinase activity, which helps separate it from most other corynebacteria. A Loeffler serum slant can be used to show metachromatic granules. Any suspected isolate must then be tested for toxin production.
- A: Modified Thayer-Martin agar is used for pathogenic Neisseria.
- B: Regan-Lowe agar is used for Bordetella pertussis.
- C: Buffered charcoal yeast extract agar is used for Legionella.
Question 4 · Easy
In a child with a sore throat, a negative rapid antigen test for group A streptococci should be:
1. ARepeated on a nasal swab
2. BAccepted as ruling out infection
3. CBacked up with a throat culture
4. DFollowed by an ASO titer
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Answer: C · Backed up with a throat culture
Rapid antigen tests are highly specific, but their sensitivity is only about 70% to 90%. In children and adolescents, who are at risk of rheumatic fever, guidelines therefore recommend a backup throat culture on sheep blood agar when the rapid antigen test is negative. A positive rapid test needs no culture. Backup culture is not needed in most adults, or after a negative result from a highly sensitive molecular test.
- A: Group A streptococci are sampled from the tonsils and posterior pharynx, not the nose.
- B: The test misses too many infections to rule out disease in children.
- D: ASO titers show past infection and do not diagnose acute pharyngitis.
Question 5 · Hard
A 17-year-old has pharyngitis and a fine rash on the trunk, and the rapid group A strep test is negative. At 48 hours the throat culture shows small beta-hemolytic colonies of catalase-negative, irregular gram-positive rods, and the reverse CAMP test is positive. What is the organism?
1. ACorynebacterium diphtheriae
2. BStreptococcus pyogenes
3. CArcanobacterium haemolyticum
4. DListeria monocytogenes
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Answer: C · Arcanobacterium haemolyticum
Arcanobacterium haemolyticum causes pharyngitis, often with a scarlet fever-like rash, mainly in teenagers and young adults. It is a catalase-negative, irregular gram-positive rod whose small colonies show beta-hemolysis that becomes clearer at 48 hours. Its phospholipase D inhibits the beta-lysin of Staphylococcus aureus, giving a positive reverse CAMP test. It is missed if throat cultures are screened only for group A streptococci.
- A: Corynebacterium diphtheriae is catalase positive.
- B: Streptococcus pyogenes is a gram-positive coccus in chains, not a rod.
- D: Listeria is catalase positive and gives a standard, not reverse, CAMP reaction.
Question 6 · Hard
A 35-year-old, last vaccinated against pertussis more than 10 years ago, has had paroxysmal cough with post-tussive vomiting for 6 weeks. Which test is most likely to confirm pertussis now?
1. ADirect fluorescent antibody on nasal secretions
2. BNasopharyngeal culture on Regan-Lowe agar
3. CNasopharyngeal PCR for B. pertussis
4. DSerum IgG antibody to pertussis toxin
Answer and explanation in LearnFree account
Question 7 · Medium
When a throat culture is plated on sheep blood agar, the agar is stabbed with the loop in several places. What is the purpose?
1. ATo show hemolysis by oxygen-labile streptolysin O
2. BTo show hemolysis by oxygen-stable streptolysin S
3. CTo prevent the growth of oral flora
4. DTo isolate anaerobes from the tonsils
Answer and explanation in LearnFree account
Question 8 · Easy
Which specimen is preferred for Bordetella pertussis culture or PCR?
1. AExpectorated sputum sample
2. BNasopharyngeal swab or aspirate
3. CThroat (oropharyngeal) swab
4. DAnterior nares swab
Answer and explanation in LearnFree account
Question 9 · Easy
Which complication follows group A streptococcal pharyngitis but not streptococcal skin infection?
1. AAcute rheumatic fever
2. BStreptococcal toxic shock
3. CAcute glomerulonephritis
4. DNecrotizing fasciitis
Answer and explanation in LearnFree account
Question 10 · Medium
Large beta-hemolytic colonies from a throat culture are catalase-negative gram-positive cocci in chains. They are PYR negative and type as Lancefield group G. The organism is most likely:
1. AStreptococcus pyogenes
2. BStreptococcus agalactiae
3. CStreptococcus anginosus group
4. DStreptococcus dysgalactiae
Answer and explanation in LearnFree account
Question 11 · Medium
Small gram-negative coccobacilli grow on chocolate agar but not on sheep blood agar and need both X and V factors. What is the organism?
1. AHaemophilus parainfluenzae
2. BBordetella pertussis
3. CPasteurella multocida
4. DHaemophilus influenzae
Answer and explanation in LearnFree account

Upper respiratory tract

After Corynebacterium diphtheriae is isolated from a throat swab, the strain's ability to cause diphtheria is shown by:

A throat swab for suspected diphtheria is plated on blood agar and on:

In a child with a sore throat, a negative rapid antigen test for group A streptococci should be:

A 17-year-old has pharyngitis and a fine rash on the trunk, and the rapid group A strep test is negative. At 48 hours the throat culture shows small beta-hemolytic colonies of catalase-negative, irregular gram-positive rods, and the reverse CAMP test is positive. What is the organism?

A 35-year-old, last vaccinated against pertussis more than 10 years ago, has had paroxysmal cough with post-tussive vomiting for 6 weeks. Which test is most likely to confirm pertussis now?

When a throat culture is plated on sheep blood agar, the agar is stabbed with the loop in several places. What is the purpose?

Which specimen is preferred for Bordetella pertussis culture or PCR?

Which complication follows group A streptococcal pharyngitis but not streptococcal skin infection?

Large beta-hemolytic colonies from a throat culture are catalase-negative gram-positive cocci in chains. They are PYR negative and type as Lancefield group G. The organism is most likely:

Small gram-negative coccobacilli grow on chocolate agar but not on sheep blood agar and need both X and V factors. What is the organism?