You’re cruising through a Q-bank, feeling good, and then a child with a “sore throat” question nukes your confidence because every option sounds kind of right. Corynebacterium diphtheriae questions are built to test whether you recognize the clinical syndrome and whether you understand what makes the classic wrong answers wrong. Let’s break it down the way Step expects: vignette → diagnosis → mechanism → management → systematic distractor demolition.
Tag: Microbiology > Gram-Positive Bacteria
The Clinical Vignette (Classic Q-Bank Style)
A 7-year-old who is underimmunized presents with fever, malaise, and sore throat. Exam shows gray pharyngeal pseudomembrane with cervical lymphadenopathy (“bull neck”). When the clinician attempts to scrape the membrane, it bleeds. A few days later, the child develops palpitations and weakness.
Most likely organism?
➡️ Corynebacterium diphtheriae
Why the Correct Answer Is Correct
Key ID Features (the “can’t-miss” clues)
- Gray/white pseudomembrane on tonsils/pharynx (fibrin + necrotic epithelium + inflammatory cells)
- Bleeds when scraped (membrane is adherent)
- Bull neck from cervical LAD + soft tissue edema
- Systemic toxicity after local infection: especially
- Myocarditis (arrhythmias, heart block, cardiomyopathy)
- Neuropathy (CN palsies, peripheral neuropathy)
Micro Lab Pearls (Step-friendly)
- Gram-positive pleomorphic rods (“club-shaped”)
- Metachromatic (volutin) granules
- Chinese-letter / V-shaped arrangement
- Grows on Löffler medium and tellurite agar (black colonies on tellurite)
Virulence: The Diphtheria Toxin (the real point)
Only toxigenic strains cause classic disease. The toxin is encoded by a β-prophage (lysogenic conversion).
Mechanism: A-B exotoxin
- B subunit binds host cells
- A subunit ADP-ribosylates EF-2 → inhibits protein synthesis → cell death
You’ll often see this written as:
High-yield association:
- Same core mechanism as Pseudomonas exotoxin A (both hit EF-2)
Management & Prevention (Where Qs Love to Pivot)
Immediate treatment (don’t wait for culture)
- Diphtheria antitoxin (neutralizes unbound toxin; won’t fix damage already done)
- Erythromycin or penicillin G (eradicates bacteria, stops more toxin production)
- Isolation + notify public health
- Vaccinate after recovery (infection doesn’t guarantee immunity)
Prevention
- DTaP/Tdap vaccine uses diphtheria toxoid (inactivated toxin → antibody response)
- If exposure: give antibiotic prophylaxis + update immunization status as needed
Step tip: If you see pseudomembrane + systemic effects, answer choices about “supportive care only” or “incision and drainage” are traps—this is a toxin-mediated emergency.
Q-Bank Breakdown: Why Each Distractor Is Wrong (and What It Would Look Like Instead)
Below are common Gram-positive and “upper airway” distractors that show up in diphtheria stems.
Quick Compare Table
| Answer choice | Key clue it’s that bug | Why it’s not diphtheria |
|---|---|---|
| Streptococcus pyogenes (GAS) | Exudative pharyngitis, tender anterior cervical nodes; scarlet fever rash | Exudate ≠ adherent bleeding pseudomembrane; no toxin-mediated myocarditis pattern |
| Staphylococcus aureus | Abscesses, empyema, pneumonia post-influenza; toxic shock | Doesn’t cause classic pharyngeal pseudomembrane with bleeding; different toxin syndromes |
| Streptococcus pneumoniae | Lobar pneumonia, otitis media, meningitis; asplenia risk | Not a pseudomembrane throat organism |
| Haemophilus influenzae type b | Epiglottitis (drooling, tripod), meningitis | Not Gram-positive; epiglottitis is airway emergency but no adherent pseudomembrane |
| Bordetella pertussis | Paroxysmal cough, inspiratory whoop, post-tussive emesis | No pseudomembrane; respiratory cilia toxin effects instead |
| EBV (infectious mononucleosis) | Posterior cervical LAD, hepatosplenomegaly; ampicillin rash | Exudative tonsillitis can mimic, but no gray adherent membrane + bleeding; heterophile Ab+ |
Let’s go one-by-one with the classic “examiner logic.”
Distractor 1: Streptococcus pyogenes (Group A Strep)
Why they want you to pick it: It’s the most familiar cause of sore throat, fever, and exudates.
What GAS pharyngitis actually looks like
- Fever, sore throat, tonsillar exudates
- Tender anterior cervical lymphadenopathy
- Palatal petechiae
- ± scarlatiniform rash (scarlet fever) from erythrogenic toxins
What makes it wrong here
- Diphtheria causes an adherent pseudomembrane that bleeds on removal.
- GAS exudate is typically removable without bleeding.
- The classic dangerous systemic sequel in the stem is myocarditis/neuropathy from diphtheria toxin, not rheumatic fever weeks later.
High-yield add-on: GAS is bacitracin sensitive, PYR positive, β-hemolytic.
Distractor 2: Staphylococcus aureus
Why it tempts people: “Toxin + systemic symptoms” triggers TSS associations.
What S. aureus would point to
- Skin abscesses (purulent)
- Pneumonia after influenza
- Osteomyelitis, septic arthritis
- Toxin syndromes:
- Toxic shock syndrome (superantigen) → fever, hypotension, diffuse rash, desquamation
- Scalded skin syndrome (exfoliative toxin)
- Food poisoning (preformed toxin → rapid vomiting)
Why it’s wrong here
- No classic pharyngeal pseudomembrane picture.
- Diphtheria is a localized throat infection with a membrane + systemic toxin effects; S. aureus is not the classic cause of that syndrome.
High-yield add-on: S. aureus is coagulase positive, catalase positive, often MRSA via altered PBP (mecA).
Distractor 3: Streptococcus pneumoniae
Why it’s included: It’s a common Gram-positive organism students over-select reflexively.
What S. pneumoniae actually screams
- Lobar pneumonia (rust-colored sputum)
- Otitis media, sinusitis
- Meningitis (esp. adults)
- Sepsis in asplenic patients
Why it’s wrong here
- Not a pseudomembrane pharyngitis organism.
- Wouldn’t explain adherent membrane + bleeding + “bull neck.”
High-yield add-on: Lancet-shaped diplococci, α-hemolytic, optochin sensitive, bile soluble, capsule is major virulence.
Distractor 4: Haemophilus influenzae type b (Hib)
Why it’s a favorite trap: Upper airway + unvaccinated kid = think Hib.
What Hib epiglottitis looks like
- High fever, drooling, dysphagia
- Muffled voice, tripod positioning
- Thumbprint sign on lateral neck X-ray
- Airway management is priority (don’t agitate the child)
Why it’s wrong here
- Diphtheria gives a pharyngeal pseudomembrane, not an inflamed epiglottis.
- Hib is Gram-negative coccobacillus, not Gram-positive.
- Bleeding on scraping is a membrane clue, not epiglottitis.
High-yield add-on: Hib virulence is PRP capsule; vaccine is conjugate (polysaccharide linked to protein carrier).
Distractor 5: Bordetella pertussis
Why it shows up: Another toxin-related vaccine-preventable pediatric pathogen.
What pertussis looks like
- Catarrhal phase → coughing fits
- Paroxysmal cough, inspiratory “whoop”
- Post-tussive emesis
- Leukocytosis with lymphocytosis
Why it’s wrong here
- Pertussis doesn’t cause a pseudomembrane or bleeding on removal.
- Disease is primarily due to toxins affecting cilia and immune response, not a local necrotizing membrane.
High-yield add-on: Diagnosis with PCR; treat with macrolide; prevent with acellular pertussis vaccine (DTaP).
Distractor 6: EBV (Infectious Mononucleosis)
Why it tricks people: EBV can cause dramatic tonsillar exudates.
What EBV would add
- Posterior cervical lymphadenopathy
- Hepatosplenomegaly
- Fatigue
- Heterophile antibody (Monospot)+
- Rash after amoxicillin/ampicillin
Why it’s wrong here
- The “membrane that bleeds when scraped” + bull neck + myocarditis/neuropathy points to diphtheria toxin, not EBV.
- EBV is exudative tonsillitis; it’s not classically described as an adherent pseudomembrane with bleeding.
High-Yield USMLE Takeaways (What You’re Really Being Tested On)
If you see this… think diphtheria
- Underimmunized + sore throat + gray pseudomembrane + bleeds on removal
- Bull neck
- Later myocarditis or neuropathy
Core mechanisms to memorize
- β-prophage encodes toxin (lysogenic conversion)
- Toxin ADP-ribosylates EF-2 → inhibits protein synthesis
- Treatment requires antitoxin + antibiotic
Exam “moves”
- Don’t scrape the membrane aggressively (bleeding + potential airway compromise)
- Antitoxin first, because antibiotics alone don’t neutralize circulating toxin
- Vaccination is toxoid-based and still needed after infection
Rapid-Fire Self-Check (1-minute recall)
- Gram stain/shape: Gram+ pleomorphic club-shaped rods, Chinese letters
- Special feature: Metachromatic granules
- Toxin: A-B exotoxin; hits EF-2
- Major complications: Myocarditis, neuropathy
- Treatment: Antitoxin + erythromycin/penicillin + isolation
- Prevention: DTaP/Tdap toxoid