A real-time PCR for norovirus is run on a stool specimen, and the amplification curves for the patient and the run controls are shown. The internal control (IC) is valid when its Ct is within 3 cycles of the IC Ct in the negative control. How should the patient result be reported?

Invalid; re-extract and repeat the test

Run failure; repeat all the specimens

Norovirus detected, with a high Ct

During peak influenza season, a hospitalized 70-year-old with fever, cough and myalgia has a negative rapid influenza antigen test. What is the best next step?

Test the specimen by influenza RT-PCR

Report that influenza has been ruled out

Order influenza antibody titers

Repeat the rapid antigen test on the same swab

The preferred test for diagnosing herpes simplex encephalitis is:

Viral culture of cerebrospinal fluid

Within 2 days, 18 residents and 5 staff members at a nursing home develop sudden vomiting and watery diarrhea lasting 1 to 3 days. Routine bacterial stool cultures are negative. Which test should be done next?

C. difficile toxin test on stool

Adenovirus 40/41 antigen on stool

A kidney transplant recipient is monitored for cytomegalovirus reactivation. Which test is most useful for this?

Quantitative CMV DNA PCR on plasma

Viral culture of a urine sample

CMV IgG antibody level in serum

Virology

11 ASCP MLS practice questions on Virology, 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 · Medium
A real-time PCR for norovirus is run on a stool specimen, and the amplification curves for the patient and the run controls are shown. The internal control (IC) is valid when its Ct is within 3 cycles of the IC Ct in the negative control. How should the patient result be reported?
Figure: MedExams
1. ANorovirus not detected
2. BRun failure; repeat all the specimens
3. CNorovirus detected, with a high Ct
4. DInvalid; re-extract and repeat the test
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Answer: D · Invalid; re-extract and repeat the test
The positive control amplifies (Ct about 25) and the negative control shows only its internal control (Ct about 29), so the run is valid. In the patient well the target did not amplify, but the internal control crossed the threshold only at about cycle 37, 8 cycles later than in the negative control, with a low plateau. That delay means something in the stool inhibited the reaction, so a negative target result cannot be trusted. Report the result as invalid, and re-extract or dilute the extract before repeating.
- A: A target-negative result is valid only when the internal control amplifies normally.
- B: Both run controls behaved correctly, so only this specimen is affected.
- C: The late curve is the internal control, not the norovirus target.
Question 2 · Medium
During peak influenza season, a hospitalized 70-year-old with fever, cough and myalgia has a negative rapid influenza antigen test. What is the best next step?
1. AReport that influenza has been ruled out
2. BOrder influenza antibody titers
3. CRepeat the rapid antigen test on the same swab
4. DTest the specimen by influenza RT-PCR
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Answer: D · Test the specimen by influenza RT-PCR
Rapid antigen tests are highly specific but only moderately sensitive, so they miss many cases. When influenza is common, a negative result in a patient with typical illness is often false negative, and IDSA advises molecular testing such as RT-PCR for hospitalized patients. Molecular assays are far more sensitive and also separate influenza A from B. Antiviral treatment in a high-risk patient should not wait for the result.
- A: A negative antigen test does not rule out influenza, especially when prevalence is high.
- B: Antibody titers need paired sera weeks apart and do not help acute care.
- C: Repeating an insensitive test on the same specimen adds little.
Question 3 · Easy
The preferred test for diagnosing herpes simplex encephalitis is:
1. AHSV PCR on cerebrospinal fluid
2. BViral culture of cerebrospinal fluid
3. CHSV culture of a throat swab
4. DHSV IgG and IgM on serum
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Answer: A · HSV PCR on cerebrospinal fluid
PCR for HSV DNA in CSF is the test of choice for herpes encephalitis, with high sensitivity and specificity and results within hours. Culture of CSF is almost always negative because little infectious virus is present in the fluid. Serum antibodies mainly reflect past infection, since most adults are seropositive for HSV. A PCR done in the first 72 hours can be falsely negative, so a repeat is considered if suspicion stays high.
- B: CSF culture is rarely positive in HSV encephalitis.
- C: Oral shedding of HSV is common and does not prove brain infection.
- D: Serum antibodies mainly show past infection and do not diagnose encephalitis.
Question 4 · Medium
Within 2 days, 18 residents and 5 staff members at a nursing home develop sudden vomiting and watery diarrhea lasting 1 to 3 days. Routine bacterial stool cultures are negative. Which test should be done next?
1. AC. difficile toxin test on stool
2. BNorovirus RT-PCR on stool
3. CRotavirus antigen EIA on stool
4. DAdenovirus 40/41 antigen on stool
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Answer: B · Norovirus RT-PCR on stool
Norovirus is the leading cause of gastroenteritis outbreaks in nursing homes, cruise ships and other closed settings. It spreads easily from person to person and by contaminated surfaces, so staff are often affected, and illness typically lasts 1 to 3 days with prominent vomiting. It does not grow in routine cell culture, so RT-PCR on stool is the test of choice. Outbreaks are reported to public health, and surfaces are cleaned with a bleach-based disinfectant because alcohol works poorly against it.
- A: C. difficile rarely affects healthy staff and does not cause short vomiting outbreaks.
- C: Rotavirus mainly affects young children, and vaccination has made it less common.
- D: Adenovirus 40/41 mainly causes diarrhea in young children.
Question 5 · Medium
A kidney transplant recipient is monitored for cytomegalovirus reactivation. Which test is most useful for this?
1. ACMV IgM antibody in serum
2. BViral culture of a urine sample
3. CQuantitative CMV DNA PCR on plasma
4. DCMV IgG antibody level in serum
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Answer: C · Quantitative CMV DNA PCR on plasma
Quantitative PCR measures CMV DNA in plasma or whole blood, and rising levels identify reactivation early, guide preemptive antiviral therapy and track response. Results should be reported in IU/mL calibrated to the WHO international standard, using the same assay and specimen type for serial tests. Antibody tests are unreliable in transplant recipients, who are immunosuppressed and often already seropositive. CMV is often shed in urine without causing disease, so a positive urine culture does not show active infection.
- A: IgM is often absent or late in immunosuppressed patients.
- B: Urine shedding is common and does not indicate CMV disease.
- D: CMV IgG shows past infection and is used for pretransplant risk, not monitoring.
Question 6 · Easy
Why must a nasopharyngeal specimen for direct fluorescent antibody (DFA) testing contain enough columnar epithelial cells?
1. AInfectious virus must be grown on the patient's own cells
2. BThe cells supply the enzymes that amplify viral RNA
3. CThe viral antigens are detected inside infected cells
4. DThe test detects host antibodies bound to the cells
Answer and explanation in LearnFree account
Question 7 · Hard
A 30-year-old has a cluster of painful vesicles on the lower back. A Tzanck smear of a scraped vesicle base shows multinucleated giant cells with nuclear molding. What does this result show?
1. AHSV or VZV; the smear cannot tell them apart
2. BHSV-2, because the lesions are clustered
3. CVZV, because the lesions are on the trunk
4. DCMV, because giant cells are present
Answer and explanation in LearnFree account
Question 8 · Easy
In infants, the most common cause of bronchiolitis is:
1. AParainfluenza virus
2. BRespiratory syncytial virus
3. CAdenovirus
4. DInfluenza B virus
Answer and explanation in LearnFree account
Question 9 · Hard
In August, a 7-year-old has fever, headache and a stiff neck. CSF: WBC 95/µL (95 x 10^6/L) with 75% lymphocytes, glucose 58 mg/dL (3.2 mmol/L), protein 52 mg/dL (520 mg/L), Gram stain negative. Which test is most likely to confirm the cause?
1. AHSV-2 PCR on CSF
2. BEnterovirus RT-PCR on CSF
3. CCryptococcal antigen on CSF
4. DBacterial culture of CSF
Answer and explanation in LearnFree account
Question 10 · Easy
Antibiotics and an antifungal agent are added to viral transport medium to:
1. ASuppress bacteria and fungi in the specimen
2. BLyse host cells so that virus is released
3. CProtect viral nucleic acid from nucleases
4. DStop the virus from multiplying in transit
Answer and explanation in LearnFree account
Question 11 · Medium
How does the shell vial technique detect cytomegalovirus faster than conventional tube culture?
1. AIt amplifies viral DNA from the culture fluid
2. BIt stains early antigens before CPE appears
3. CIt detects CMV IgM in the patient's serum
4. DIt waits for focal CPE in human fibroblasts
Answer and explanation in LearnFree account

Virology

A real-time PCR for norovirus is run on a stool specimen, and the amplification curves for the patient and the run controls are shown. The internal control (IC) is valid when its Ct is within 3 cycles of the IC Ct in the negative control. How should the patient result be reported?

During peak influenza season, a hospitalized 70-year-old with fever, cough and myalgia has a negative rapid influenza antigen test. What is the best next step?

The preferred test for diagnosing herpes simplex encephalitis is:

Within 2 days, 18 residents and 5 staff members at a nursing home develop sudden vomiting and watery diarrhea lasting 1 to 3 days. Routine bacterial stool cultures are negative. Which test should be done next?

A kidney transplant recipient is monitored for cytomegalovirus reactivation. Which test is most useful for this?

Why must a nasopharyngeal specimen for direct fluorescent antibody (DFA) testing contain enough columnar epithelial cells?

A 30-year-old has a cluster of painful vesicles on the lower back. A Tzanck smear of a scraped vesicle base shows multinucleated giant cells with nuclear molding. What does this result show?

In infants, the most common cause of bronchiolitis is:

In August, a 7-year-old has fever, headache and a stiff neck. CSF: WBC 95/µL (95 x 10^6/L) with 75% lymphocytes, glucose 58 mg/dL (3.2 mmol/L), protein 52 mg/dL (520 mg/L), Gram stain negative. Which test is most likely to confirm the cause?

Antibiotics and an antifungal agent are added to viral transport medium to:

How does the shell vial technique detect cytomegalovirus faster than conventional tube culture?