Medical Laboratory Technician Clinical Microbiology Questions and Answers — Questions and Answers
Question 1: A Gram stain performed on a purulent aspirate from a skin abscess reveals gram-positive cocci arranged in grape-like clusters. Which of the following organisms is most likely responsible for the infection?
- Streptococcus pyogenes
- Enterococcus faecalis
- Staphylococcus aureus (Correct answer)
- Streptococcus pneumoniae
Correct answer: Staphylococcus aureus
*Staphylococcus aureus* is classically described as forming grape-like clusters and is a primary cause of skin and soft tissue infections, including abscesses. *Streptococcus* and *Enterococcus* species typically arrange themselves in pairs and chains.
Question 2: MacConkey agar is a selective and differential medium used for isolating enteric bacteria. The selective agents that inhibit the growth of most gram-positive organisms are:
- A high concentration of sodium chloride
- Bile salts and crystal violet (Correct answer)
- Colistin and nalidixic acid
- Phenylethyl alcohol
Correct answer: Bile salts and crystal violet
MacConkey agar contains bile salts and crystal violet, which effectively inhibit the growth of most gram-positive bacteria, thus selecting for gram-negative organisms. A high salt concentration is used in Mannitol Salt Agar, CNA is used in Columbia CNA agar, and phenylethyl alcohol is used in PEA agar.
Question 3: A Kirby-Bauer susceptibility test for a *Staphylococcus aureus* isolate shows resistance to erythromycin and susceptibility to clindamycin, but with a flattening of the clindamycin zone of inhibition adjacent to the erythromycin disk. This characteristic "D" shape indicates:
- Beta-lactamase production
- Intrinsic resistance to vancomycin
- Tolerance to the antimicrobial agent
- Inducible clindamycin resistance (Correct answer)
Correct answer: Inducible clindamycin resistance
The D-test is performed to detect inducible clindamycin resistance, mediated by the *erm* gene. Erythromycin induces the expression of this gene, causing the organism to become resistant to clindamycin in the area near the erythromycin disk, resulting in a "D-shaped" zone of inhibition. Failure to detect this can lead to clindamycin treatment failure.
Question 4: A patient presents with prolonged, foul-smelling, fatty diarrhea (steatorrhea) after a camping trip where they drank unfiltered stream water. A microscopic examination of a stool sample is most likely to reveal which of the following?
- Pear-shaped, binucleated trophozoites (Correct answer)
- Lemon-shaped cysts with a polar plug
- Acid-fast positive oocysts
- Flask-shaped ulcers upon colonoscopy
Correct answer: Pear-shaped, binucleated trophozoites
The clinical history and symptoms are classic for giardiasis, caused by *Giardia lamblia*. The diagnostic form in a diarrheic stool specimen is the pear-shaped, binucleated trophozoite with characteristic 'falling leaf' motility.
Question 5: An India ink preparation is performed on a cerebrospinal fluid (CSF) sample from an immunocompromised patient with suspected meningitis. This technique is used to visualize which feature for the presumptive identification of *Cryptococcus neoformans*?
- Germ tubes
- The polysaccharide capsule (Correct answer)
- Arthroconidia
- Chlamydospores
Correct answer: The polysaccharide capsule
*Cryptococcus neoformans* is characterized by a large polysaccharide capsule, which is a major virulence factor. The India ink stain is a negative staining method where the ink particles are excluded by the capsule, creating a distinct, clear halo around the yeast cell against a dark background.
Question 6: A gram-negative rod isolated from a urine culture grows as dry, pink colonies on MacConkey agar. The following biochemical test results are obtained: Indole: Positive Methyl Red: Positive Voges-Proskauer: Negative Citrate: Negative What is the most probable identification of this organism?
- Klebsiella pneumoniae
- Proteus mirabilis
- Pseudomonas aeruginosa
- Escherichia coli (Correct answer)
Correct answer: Escherichia coli
This classic IMViC (Indole, Methyl Red, Voges-Proskauer, Citrate) reaction pattern of ++-- is characteristic of *Escherichia coli*. The organism's ability to ferment lactose (pink colonies on MacConkey) further supports this identification.
Medical Laboratory Technician Clinical Microbiology Questions and Answers
A Gram stain performed on a purulent aspirate from a skin abscess reveals gram-positive cocci arranged in grape-like clusters.
Which of the following organisms is most likely responsible for the infection?