Dental MCQ
Gram-positive and Gram-negative · Microbiology · INBDE Patient Cases

Bacterial Infections INBDE Patient Cases

By Dr. Isaac Sun, DDS

9 ADA INBDE-format patient cases on bacterial infections. Each case is a shared patient box (chief complaint, history, medications, allergies, exam) followed by linked multiple-choice questions with full distractor explanations. Practice the way the real exam is structured.

9 patient cases45 linked questionsADA INBDE formatFull distractor explanations

Nine ADA INBDE-format patient cases connecting bacterial infections to dentistry: viridans streptococcal bacteremia and infective endocarditis prophylaxis in a prosthetic-valve patient, cervicofacial actinomycosis (lumpy jaw) with sulfur granules and Actinomyces israelii, Group A streptococcal scarlet fever with strawberry tongue and the rheumatic fever link to valve damage, primary syphilis and the painless oral chancre of Treponema pallidum with its infectiousness, tetanus versus odontogenic trismus with Clostridium tetani and tetanospasmin, Pseudomonas aeruginosa biofilm in dental unit waterlines and the risk to an immunocompromised patient, Clostridioides difficile colitis after clindamycin and the antibiotic-stewardship lesson, a Staphylococcus aureus perioral abscess with coagulase-positivity and MRSA awareness, and a non-healing tongue ulcer raising oral tuberculosis. Topics include Gram-positive cocci (streptococci and staphylococci), odontogenic anaerobes and actinomycosis, Gram-negative bacteria, spirochetes and mycobacteria (syphilis and tuberculosis), and toxin-mediated and antibiotic-associated infections.

Case Coverage Map
What each case is testing
Extraction in a patient with a prosthetic valve:
Viridans strep bacteremia, the highest-risk cardiac group, amoxicillin prophylaxis, oral health to limit bacteremia, and anticoagulation.
A chronic lumpy jaw swelling that keeps draining:
Cervicofacial actinomycosis, sulfur granules, Actinomyces as a filamentous anaerobe, and the need for prolonged antibiotics plus drainage.
Sore throat, strawberry tongue, and a rash:
Group A strep scarlet fever, the rheumatic fever link to valve damage, and the dentist's recognize-and-refer role.
A painless ulcer on the lip:
Primary syphilis chancre, Treponema pallidum, infectiousness and standard precautions, progression to secondary syphilis, and referral.
Lockjaw after a dirty wound:
Tetanus versus odontogenic trismus, Clostridium tetani and tetanospasmin (an exotoxin), emergency referral, and toxoid prevention.
Dental unit waterlines and a vulnerable patient:
Pseudomonas aeruginosa biofilm in waterlines, risk in neutropenia, why lines get contaminated, maintenance, and oncology coordination.
Diarrhea after antibiotics for a tooth:
Clostridioides difficile colitis after clindamycin, the flora-disruption mechanism, its toxins, referral, and antibiotic stewardship.
A recurring boil at the corner of the mouth:
Staphylococcus aureus skin abscess, coagulase-positivity, drainage as primary treatment, MRSA awareness, and central-face caution.
A non-healing ulcer on the tongue:
Oral tuberculosis in the differential, Mycobacterium tuberculosis, airborne precautions, and biopsy and referral for a non-healing ulcer.
Patient case: Extraction in a patient with a prosthetic valve
0 of 5 answered, 0 correct
Patient
Male, 63 years old
Chief Complaint
"I need this broken tooth out, and I have a heart valve replacement."
Background and/or Patient History
  • Mechanical prosthetic aortic valve placed years ago
  • Presents for extraction of a non-restorable molar
  • No current fever or systemic symptoms
  • Takes a blood thinner
Allergies
NKDA
Medications
  • Warfarin
Current Findings
  • Non-restorable molar with chronic apical pathology
  • Otherwise stable; no signs of active infection
  1. Question 1
    Moderate
    The organisms most likely to be seeded into his blood during the extraction are:
  2. Question 2
    Moderate
    His prosthetic valve places him in which category for endocarditis prophylaxis?
  3. Question 3
    Moderate
    The standard first-line prophylactic antibiotic (if he can take it) before the extraction is:
  4. Question 4
    Moderate
    Besides the single pre-procedure dose, what most reduces his day-to-day endocarditis risk?
  5. Question 5
    Moderate
    His warfarin is relevant to the extraction because the dentist should:

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Patient case: A chronic lumpy jaw swelling that keeps draining
0 of 5 answered, 0 correct
Patient
Male, 50 years old
Chief Complaint
"This lump on my jaw won't go away and keeps oozing."
Background and/or Patient History
  • Firm jaw swelling for several weeks after a difficult extraction
  • A small opening on the skin intermittently drains pus
  • Took a short course of antibiotics with little change
  • Otherwise healthy
Allergies
NKDA
Medications
  • None
Current Findings
  • Indurated submandibular swelling with a draining sinus tract
  • Drainage contains tiny yellow granules
  1. Question 1
    Moderate
    The chronic indurated swelling with a draining sinus and yellow granules is most consistent with:
  2. Question 2
    Moderate
    The yellow granules in the drainage are:
  3. Question 3
    Hard
    Actinomyces israelii is best classified as a:
  4. Question 4
    Moderate
    Why did the short antibiotic course fail to clear it?
  5. Question 5
    Moderate
    Appropriate management now is:

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Patient case: Sore throat, strawberry tongue, and a rash
0 of 5 answered, 0 correct
Patient
Female, 8 years old, accompanied by parent
Chief Complaint
Parent: "She has a bad sore throat, a rash, and her tongue looks strange."
Background and/or Patient History
  • Two days of fever and severe sore throat
  • A fine sandpaper-like rash on the trunk
  • No cough or runny nose
  • Up to date on routine vaccinations
Allergies
NKDA
Medications
  • None
Current Findings
  • Red, swollen tonsils; palatal petechiae
  • Bright red, bumpy 'strawberry' tongue
  1. Question 1
    Moderate
    Her sore throat with a strawberry tongue, palatal petechiae, and sandpaper rash points to:
  2. Question 2
    Moderate
    The causative organism is:
  3. Question 3
    Moderate
    Why is it important that untreated Group A streptococcal pharyngitis be treated?
  4. Question 4
    Hard
    If she later developed rheumatic heart disease with valve damage, that history would matter in dentistry because it may:
  5. Question 5
    Moderate
    The dentist's appropriate role on noticing these signs is to:

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Patient case: A painless ulcer on the lip
0 of 5 answered, 0 correct
Patient
Male, 31 years old
Chief Complaint
"I have a sore on my lip that doesn't hurt and won't heal."
Background and/or Patient History
  • A firm, painless ulcer on the lip for about three weeks
  • No pain, which he finds odd
  • Came in mainly for a cleaning
  • Otherwise feels well
Allergies
NKDA
Medications
  • None
Current Findings
  • Solitary, indurated, painless ulcer on the lip
  • Nontender enlarged regional lymph node
  1. Question 1
    Moderate
    A solitary, firm, painless oral ulcer with regional lymphadenopathy should raise suspicion for:
  2. Question 2
    Easy
    The causative organism is:
  3. Question 3
    Moderate
    An important infection-control point about oral syphilitic lesions is that they are:
  4. Question 4
    Moderate
    Untreated, this would likely progress to secondary syphilis, which can show:
  5. Question 5
    Moderate
    The dentist's appropriate next step is to:

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Patient case: Lockjaw after a dirty wound
0 of 5 answered, 0 correct
Patient
Male, 57 years old
Chief Complaint
"My jaw is getting stiff and hard to open, and my whole body feels tight."
Background and/or Patient History
  • Stepped on a nail in the garden about a week ago
  • Cannot recall his last tetanus booster
  • Now has worsening jaw stiffness and difficulty opening
  • Also reports neck and back muscle stiffness
Allergies
NKDA
Medications
  • None
Current Findings
  • Marked trismus with no dental cause found
  • Generalized muscle rigidity; a healing puncture wound on the foot
  1. Question 1
    Moderate
    His trismus with generalized rigidity and a recent contaminated wound should raise concern for:
  2. Question 2
    Moderate
    The organism responsible is:
  3. Question 3
    Hard
    The muscle spasms of tetanus are caused by a bacterial:
  4. Question 4
    Moderate
    The most appropriate action by the dental team is to:
  5. Question 5
    Moderate
    The key preventive measure against tetanus is:

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Patient case: Dental unit waterlines and a vulnerable patient
0 of 5 answered, 0 correct
Patient
Female, 66 years old
Chief Complaint
"I'm on chemotherapy. Is the water in the dental equipment safe for me?"
Background and/or Patient History
  • Undergoing chemotherapy with a low white-cell count
  • Scheduled for a cleaning and a filling
  • Concerned about infection risk
  • Her oncologist advised caution with elective procedures
Allergies
NKDA
Medications
  • Chemotherapy regimen
Current Findings
  • Immunocompromised (neutropenic)
  • No active oral infection today
  1. Question 1
    Moderate
    The waterborne, biofilm-forming organism of particular concern in dental unit waterlines is:
  2. Question 2
    Moderate
    Why is Pseudomonas in waterlines especially risky for this patient?
  3. Question 3
    Moderate
    Dental unit waterlines become contaminated mainly because:
  4. Question 4
    Moderate
    The recommended control for waterline contamination is to:
  5. Question 5
    Moderate
    Given her neutropenia, the most appropriate overall approach is to:

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Patient case: Diarrhea after antibiotics for a tooth
0 of 5 answered, 0 correct
Patient
Female, 70 years old
Chief Complaint
"Since the antibiotics for my tooth, I've had bad watery diarrhea."
Background and/or Patient History
  • Took clindamycin for a dental infection over the past week
  • Now has frequent watery diarrhea and abdominal cramps
  • Feels weak and slightly feverish
  • No blood thinners
Allergies
Penicillin
Medications
  • Recently finished clindamycin
Current Findings
  • Reports up to several loose stools a day
  • Mild lower abdominal tenderness; low-grade fever
  1. Question 1
    Moderate
    Her diarrhea after a clindamycin course is most likely:
  2. Question 2
    Moderate
    The mechanism is that the antibiotic:
  3. Question 3
    Hard
    C. difficile produces disease mainly through its:
  4. Question 4
    Moderate
    The most appropriate immediate step for the dental team is to:
  5. Question 5
    Moderate
    The broader prescribing lesson, reinforced by her penicillin allergy note, is that the dentist should:

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Patient case: A recurring boil at the corner of the mouth
0 of 5 answered, 0 correct
Patient
Male, 28 years old
Chief Complaint
"I keep getting painful pus-filled bumps near my lip."
Background and/or Patient History
  • Recurrent tender pustular lesions at the perioral skin
  • One area is now a fluctuant, pus-filled swelling
  • No dental source identified on exam
  • Otherwise healthy
Allergies
NKDA
Medications
  • None
Current Findings
  • Fluctuant perioral skin abscess with surrounding redness
  • Teeth and gingiva healthy
  1. Question 1
    Moderate
    The most likely organism in this perioral skin abscess is:
  2. Question 2
    Moderate
    A laboratory feature that identifies S. aureus is that it is:
  3. Question 3
    Moderate
    The single most important treatment for the fluctuant abscess is:
  4. Question 4
    Hard
    If empiric antibiotics are needed and community resistance is a concern, the team must consider:
  5. Question 5
    Moderate
    An important caution with infections in the central face (around the nose and upper lip) is the risk of:

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Patient case: A non-healing ulcer on the tongue
0 of 5 answered, 0 correct
Patient
Male, 54 years old
Chief Complaint
"This sore on my tongue has been there for over a month and won't heal."
Background and/or Patient History
  • Persistent tongue ulcer for over six weeks
  • Several weeks of cough, night sweats, and weight loss
  • Recently immigrated from a region with high tuberculosis prevalence
  • Has not seen a physician
Allergies
NKDA
Medications
  • None
Current Findings
  • A chronic, irregular ulcer on the tongue with no clear trauma
  • Appears thin and fatigued; chronic cough
  1. Question 1
    Moderate
    Given his systemic symptoms and a non-healing tongue ulcer, one important infectious cause to consider is:
  2. Question 2
    Easy
    The causative organism would be:
  3. Question 3
    Moderate
    Because he likely has active pulmonary tuberculosis, the most relevant infection-control measure is:
  4. Question 4
    Moderate
    To establish the cause of the chronic ulcer itself, the appropriate step is:
  5. Question 5
    Easy
    This case reinforces that a non-healing oral ulcer is:

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