Diphtheria Practice Questions
20 free Diphtheria practice questions for the Microbiology. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
Which of the following is the primary causative agent of Diphtheria?
- A Corynebacterium diphtheriae
- B Mycobacterium tuberculosis
- C Streptococcus pyogenes
- D Haemophilus influenzae
Correct answer: Corynebacterium diphtheriae
Corynebacterium diphtheriae is a Gram-positive, non-motile bacillus responsible for the disease. It is often characterized by its club-shaped appearance under microscopic examination.
The diphtheria toxin exerts its virulence by inhibiting which cellular process?
- A DNA replication
- B Cell wall synthesis
- C RNA transcription
- D Protein synthesis
Correct answer: Protein synthesis
The toxin acts as an ADP-ribosyltransferase that inactivates elongation factor 2 (EF-2). This cessation of protein synthesis eventually leads to cell death in the host.
What is the characteristic clinical feature of respiratory diphtheria found in the pharynx?
- A Grayish-white pseudomembrane
- B Petechial hemorrhages
- C Diffuse vesicular rash
- D Erythematous tonsillar exudate
Correct answer: Grayish-white pseudomembrane
A thick, leathery pseudomembrane composed of fibrin, dead cells, and bacteria forms over the tonsils and pharynx. Attempting to remove this membrane typically causes bleeding of the underlying tissue.
Which medium is specifically used for the primary isolation of C. diphtheriae to observe characteristic metachromatic granules?
- A Loeffler's serum slope
- B MacConkey agar
- C Thayer-Martin agar
- D Lowenstein-Jensen medium
Correct answer: Loeffler's serum slope
Loeffler's serum slope is an enriched medium that promotes the rapid growth of C. diphtheriae. It specifically enhances the development of the intracellular polyphosphate storage granules known as metachromatic granules.
The 'bull neck' appearance in severe diphtheria is primarily caused by which of the following?
- A Cervical lymphadenopathy and edema
- B Thyroid gland enlargement
- C Subcutaneous emphysema
- D Hypertrophy of the sternocleidomastoid muscle
Correct answer: Cervical lymphadenopathy and edema
Severe toxic diphtheria causes massive swelling of the cervical lymph nodes and surrounding soft tissues of the neck. Clinical progression to this state indicates a high risk of airway obstruction.
Which specialized stain is used to visualize the Volutin (metachromatic) granules of C. diphtheriae?
- A Albert's stain
- B Gros's stain
- C Ziehl-Neelsen stain
- D India ink stain
Correct answer: Albert's stain
Albert's stain is specifically designed to highlight metachromatic granules, which appear bluish-black against the green-stained cytoplasm of the bacilli. This provides a presumptive identification of the organism.
C. diphtheriae strains become toxigenic only after being infected by which of the following?
- A A self-replicating plasmid
- B A lysogenic beta-phage
- C A virulent transposon
- D An airborne fungus
Correct answer: A lysogenic beta-phage
The tox gene, which encodes the diphtheria toxin, is carried by a corynephage (beta-phage). Strains that do not carry this prophage are non-toxigenic and generally do not cause severe systemic disease.
Which laboratory test is the gold standard for demonstrating the toxigenicity of a C. diphtheriae isolate?
- A Schick intradermal skin test
- B Elek's gel precipitation test
- C Widal slide agglutination test
- D Dick erythrogenic toxin test
Correct answer: Elek's gel precipitation test
Elek's test is an in vitro immunoprecipitation assay where toxin produced by the bacteria reacts with antitoxin on a filter paper strip. The formation of visible precipitin lines confirms the production of the diphtheria toxin.
What is the primary mechanism of the Schick test?
- A To identify the presence of C. diphtheriae in the throat
- B To determine the antibiotic sensitivity of the strain
- C To assess an individual's immunity to diphtheria toxin
- D To measure the concentration of circulating bacteria
Correct answer: To assess an individual's immunity to diphtheria toxin
The Schick test involves the intradermal injection of a small amount of diphtheria toxin. A lack of skin reaction indicates the presence of neutralizing antibodies and thus immunity to the disease.
Which of the following is the most common systemic complication of diphtheria toxin?
- A Acute glomerulonephritis
- B Myocarditis
- C Arthritis
- D Splenic rupture
Correct answer: Myocarditis
The toxin has a high affinity for cardiac and nervous tissue. Myocarditis, which can lead to heart failure and arrhythmias, is a leading cause of death in patients with diphtheria.
In the context of the diphtheria vaccine (DTaP), what does the 'D' represent?
- A Attenuated live bacteria
- B Killed pathogen
- C Diphtheria toxoid
- D Purified surface protein
Correct answer: Diphtheria toxoid
The vaccine contains diphtheria toxoid, which is the diphtheria toxin inactivated by formaldehyde. It retains its immunogenicity to induce protective antibodies without causing the harmful effects of the toxin.
Which selective medium is used to differentiate C. diphtheriae by the grayish-black color of its colonies?
- A Deoxycholate citrate agar
- B Hoyle's tellurite agar
- C Chocolate agar medium
- D MacConkey agar plate
Correct answer: Hoyle's tellurite agar
On Hoyle's or Tinsdale agar, tellurite is reduced by C. diphtheriae to metallic tellurium. This results in the formation of characteristic grayish-black colonies, aiding in the differentiation from other flora.
A 5-year-old child presents with a sore throat and a thick membrane in the pharynx. What is the immediate priority in management?
- A Wait for laboratory culture confirmation
- B Administration of diphtheria antitoxin
- C Oral gargling with warm saline solution
- D Surgical removal of the membrane
Correct answer: Administration of diphtheria antitoxin
Diphtheria antitoxin should be administered immediately based on clinical suspicion, as it can only neutralize unbound toxin. Delays in administration significantly increase the risk of permanent organ damage or death.
Which biotype of C. diphtheriae is most commonly associated with severe clinical disease and outbreaks?
- A Mitis
- B Intermedius
- C Gravis
- D Belfanti
Correct answer: Gravis
C. diphtheriae is classified into four biotypes: gravis, mitis, intermedius, and belfanti. The gravis biotype is historically associated with more severe disease and higher mortality rates.
The arrangement of C. diphtheriae cells on a smear is often described as resembling which of the following?
- A Long filamentous chains
- B Chinese letters or cuneiform
- C Grape-like clusters of spheres
- D Paired safety pin forms
Correct answer: Chinese letters or cuneiform
Due to the incomplete separation of daughter cells during binary fission (snapping division), the bacilli often arrange themselves at sharp angles. This creates patterns resembling Chinese characters or the letter 'V'.
How is diphtheria primarily transmitted from person to person?
- A Fecal-oral ingestion route
- B Direct sexual contact
- C Contaminated water
- D Respiratory droplets
Correct answer: Respiratory droplets
Diphtheria is highly contagious and spreads through respiratory droplets expelled during coughing or sneezing. It can also be spread through contact with skin lesions in the case of cutaneous diphtheria.
Which antibiotic is typically used to eliminate the carrier state and stop toxin production in diphtheria patients?
- A Erythromycin
- B Ciprofloxacin
- C Amphotericin B
- D Tetracycline
Correct answer: Erythromycin
Erythromycin or Penicillin G are the antibiotics of choice to kill the bacteria and prevent further spread. While they stop toxin production, they do not neutralize toxin already present in the body.
What is the biochemical role of the 'B' subunit of the diphtheria toxin?
- A It provides the direct enzymatic activity
- B It cleaves DNA strands within the nucleus
- C It prevents the fusion of phagosomes with lysosomes
- D It facilitates binding to host cell receptors
Correct answer: It facilitates binding to host cell receptors
Diphtheria toxin is an A-B toxin. The 'B' subunit binds to the heparin-binding epidermal growth factor receptor on host cells, allowing the 'A' subunit to enter the cytoplasm.
Cutaneous diphtheria is most frequently characterized by which of the following?
- A A diffuse red rash similar to scarlet fever
- B Large fluid-filled bullae
- C Wart-like growths on the hands
- D Non-healing ulcers with a gray membrane
Correct answer: Non-healing ulcers with a gray membrane
Cutaneous diphtheria typically presents as chronic, non-healing ulcers often covered by a grayish pseudomembrane. It is more common in tropical regions and among populations with poor hygiene.
In the regulation of diphtheria toxin production, what substance acts as a co-repressor with the DtxR protein?
- A Calcium
- B Magnesium
- C Zinc
- D Iron
Correct answer: Iron
Toxin production is regulated by iron levels. When iron is plentiful, it binds to the DtxR protein to repress the toxin gene; when iron is low, the repression is lifted and toxin is produced.