Free AP-MICRO Specimen Processing and Stains Questions and Answers 1 — Questions and Answers
Question 1: A cerebrospinal fluid (CSF) specimen from a patient with suspected meningitis is received in the microbiology lab. A direct smear is prepared using India ink. The technologist observes encapsulated yeast cells. Which of the following is the most likely organism?
- Candida albicans
- Aspergillus fumigatus
- Cryptococcus neoformans (Correct answer)
- Histoplasma capsulatum
Correct answer: Cryptococcus neoformans
The India ink stain is a negative staining technique used to visualize capsules surrounding certain microorganisms. Cryptococcus neoformans is a yeast known for its prominent polysaccharide capsule, which appears as a clear halo against a dark background in an India ink preparation. This is a classic presumptive identification method for this organism in CSF.
Question 2: A Gram stain of a sputum specimen reveals >25 squamous epithelial cells and <10 polymorphonuclear cells per low-power field. What is the most appropriate action for the technologist to take?
- Inoculate the specimen onto all routine culture media.
- Request a new, more deeply expectorated specimen. (Correct answer)
- Perform an acid-fast stain on the specimen.
- Report the Gram stain results as 'normal oral flora'.
Correct answer: Request a new, more deeply expectorated specimen.
The presence of a high number of squamous epithelial cells (>10 or >25, depending on the specific lab criteria) and few polymorphonuclear cells indicates contamination with saliva and oropharyngeal flora, making the specimen unsatisfactory for culture. Processing such a specimen would likely yield misleading results. The most appropriate action is to reject the specimen and request a new, properly collected sample from the lower respiratory tract.
Question 3: In the Ziehl-Neelsen acid-fast staining procedure, what is the function of the carbolfuchsin?
- Mordant
- Decolorizer
- Counterstain
- Primary stain (Correct answer)
Correct answer: Primary stain
Carbolfuchsin serves as the primary stain in the Ziehl-Neelsen method. Due to the high mycolic acid (waxy) content in the cell walls of acid-fast organisms like Mycobacterium, heat is applied to help the carbolfuchsin penetrate the cell wall. Once stained, these organisms resist decolorization by acid-alcohol.
Question 4: A laboratory receives a tissue biopsy for fungal culture and direct examination. Which stain is a rapid and highly sensitive fluorochrome that binds to chitin and cellulose in fungal cell walls, causing them to fluoresce brightly under a UV microscope?
- Calcofluor white (Correct answer)
- Lactophenol cotton blue
- Periodic acid-Schiff (PAS)
- Gomori methenamine silver (GMS)
Correct answer: Calcofluor white
Calcofluor white is a fluorescent dye that nonspecifically binds to chitin and cellulose, which are major components of fungal cell walls. This binding causes the fungal elements to fluoresce a brilliant apple-green or blue-white color under UV light, making it a very rapid and sensitive method for detecting fungi in clinical specimens.
Question 5: A microbiologist is performing a Gram stain. After the decolorization step with alcohol/acetone, what is the state of Gram-positive and Gram-negative bacteria?
- Gram-positive are purple, Gram-negative are colorless. (Correct answer)
- Gram-positive are purple, Gram-negative are pink.
- Both are colorless.
- Both are purple.
Correct answer: Gram-positive are purple, Gram-negative are colorless.
The critical step in differentiating bacteria with the Gram stain is decolorization. Gram-positive bacteria have a thick peptidoglycan cell wall which retains the crystal violet-iodine complex, so they remain purple. Gram-negative bacteria have a thin peptidoglycan layer and an outer lipid membrane that is dissolved by the alcohol/acetone, allowing the crystal violet-iodine complex to be washed out, rendering them colorless until the counterstain is applied.
Question 6: A CSF specimen is sent to the lab for bacterial culture and must be held for one hour before processing. What is the proper storage condition for this specimen?
- Refrigerated at 2-8°C
- Frozen at -20°C
- Incubated at 35-37°C
- Kept at room temperature (20-25°C) (Correct answer)
Correct answer: Kept at room temperature (20-25°C)
CSF specimens for bacterial culture should never be refrigerated, as fastidious pathogens like Neisseria meningitidis and Haemophilus influenzae are sensitive to cold and may not survive. The specimen should be processed immediately, but if a delay is unavoidable, it should be kept at room temperature to maintain the viability of potential pathogens.
Free AP-MICRO Specimen Processing and Stains Questions and Answers 1
A cerebrospinal fluid (CSF) specimen from a patient with suspected meningitis is received in the microbiology lab.
A direct smear is prepared using India ink.
The technologist observes encapsulated yeast cells.
Which of the following is the most likely organism?