FREE AP-MICRO Specimen Processing and Stains 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?

Accepted Answer

Cryptococcus neoformans

Answer

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?

Accepted Answer

Request a new, more deeply expectorated specimen.

Answer

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?

Accepted Answer

Primary stain

Answer

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?

Accepted Answer

Calcofluor white

Answer

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?

Accepted Answer

Gram-positive are purple, Gram-negative are colorless.

Answer

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.

AP Practice Test

AP Practice Test

FREE AP-MICRO Specimen Processing and Stains Questions and Answers