AP-Micro Microbiology: Specimen Processing and Stains 2

Question 1

Which fixative is commonly used to preserve bacterial morphology before Gram staining?

Accepted Answer

Heat fixation

Answer

Heat fixation kills bacteria and adheres them to the slide without distorting morphology significantly.

Question 2

A sputum sample is collected from a patient suspected of having tuberculosis. What is the preferred transport medium?

Accepted Answer

No medium — direct smear within 1 hour

Answer

Sputum for AFB smear and culture should be processed promptly or refrigerated without transport medium to avoid overgrowth.

Question 3

The Ziehl-Neelsen stain uses carbol fuchsin as the primary stain. What is its function?

Accepted Answer

It penetrates the waxy mycolic acid cell wall when heated

Answer

Carbol fuchsin, aided by heat, penetrates the lipid-rich mycolic acid coat of mycobacteria to stain them red.

Question 4

Which decolorizer is used in the acid-fast stain procedure?

Accepted Answer

Acid-alcohol (3% HCl in 95% ethanol)

Answer

Acid-alcohol (3% HCl in 95% ethanol) removes carbol fuchsin from non-acid-fast organisms but not from mycobacteria.

Question 5

A CSF specimen arrives in the laboratory with a 4-hour delay at room temperature. Which organism is most at risk of being missed due to this delay?

Accepted Answer

Neisseria meningitidis

Answer

Neisseria meningitidis is extremely fragile and dies rapidly outside the body, making delays in CSF processing critical.

AP Practice Test