# INBDE practice questions: FK7 — Microbiology

Ten original INBDE practice questions on FK7 — Microbiology, each answered on this page with a rationale and a source.

Last updated: 2026-08-10.

## Question 1

A 34-year-old man has a grossly carious mandibular second molar and a firm, tender swelling extending below the inferior border of the mandible on that side. The radiograph shows the root apices lying below the mylohyoid line. Into which space has the infection most likely perforated?

- A. The submandibular space, because the apices lie below the mylohyoid line
- B. The sublingual space, because the apices lie above the mylohyoid line
- C. The canine space, following the route taken by maxillary anterior infections
- D. The submental space, which characteristically receives mandibular molar infection

**Answer A:** The submandibular space, because the apices lie below the mylohyoid line

When infection escapes the tooth, anatomy decides the destination, and the rule to know is the relationship of the apex to the muscle attachment: an apex lying below the mylohyoid line perforates into the submandibular space, and one lying above it perforates into the sublingual space. The stem supplies both the radiographic position and a swelling below the inferior border. B states the rule correctly but reads the radiograph backwards, since the apices are described as below the line. C imports the maxillary anterior and canine route, which tracks into the canine space and periorbital tissues, into a mandibular molar. D names a space that participates with the submandibular and sublingual spaces in Ludwig angina, but not the destination this apex position predicts.

**Common trap:** Knowing the mylohyoid rule but not checking which side of the line the apex sits on.

Source: [Contemporary Oral and Maxillofacial Surgery](https://shop.elsevier.com/books/contemporary-oral-and-maxillofacial-surgery/hupp/978-0-323-55221-9)

## Question 2

A 40-year-old woman with a necrotic mandibular molar has bilateral firm submandibular swelling, an elevated floor of mouth, drooling, difficulty swallowing and a muffled voice. She is afebrile and can still speak in short sentences. What is the immediate priority?

- A. Prescribe oral amoxicillin and review the swelling again the following morning
- B. Incise and drain the swelling under local anaesthesia in the surgery today
- C. Obtain a cone-beam scan to define the extent of the spaces involved
- D. Arrange immediate transfer today for airway assessment and hospital management

**Answer D:** Arrange immediate transfer today for airway assessment and hospital management

Bilateral involvement of the submandibular, sublingual and submental spaces is Ludwig angina, a rapidly spreading cellulitis that elevates the floor of the mouth and threatens the airway. Trismus, dysphagia, drooling, a raised floor of mouth, voice change and difficulty breathing are transfer-now findings, and a normal temperature does not soften them. A treats a threatened airway as an outpatient prescription and builds in a delay of many hours; antibiotics also never substitute for source control. B applies the right general principle — drain the collection first — in the wrong setting, since this drainage belongs where the airway can be secured. C delays transfer for an image that will not change the immediate decision in a diagnosis made clinically.

**Common trap:** Reading a normal temperature as evidence that a spreading infection is not yet urgent.

Source: [Contemporary Oral and Maxillofacial Surgery](https://shop.elsevier.com/books/contemporary-oral-and-maxillofacial-surgery/hupp/978-0-323-55221-9)

## Question 3

A 27-year-old woman reports painful ulcers that appeared two days after a dental appointment. Examination shows a cluster of small coalescing ulcers confined to one side of the hard palate, which she says began as tiny blisters. She has had recurrent lip lesions for years. What is the most likely diagnosis?

- A. Minor aphthous ulceration, provoked by the trauma of the appointment
- B. Primary herpetic gingivostomatitis, from a first exposure at that visit
- C. Recurrent intraoral herpes, from reactivation of the latent virus
- D. Herpes zoster of the maxillary division, which stops at the midline

**Answer C:** Recurrent intraoral herpes, from reactivation of the latent virus

HSV-1 remains latent in the trigeminal ganglion after primary infection and recurs as herpes labialis on the lip or, intraorally, as clustered vesicles breaking down into ulcers on keratinized, bound-down mucosa — the hard palate and attached gingiva. That location rule, with her history of recurrent lip lesions, is what defeats A: aphthous ulcers favour movable, non-keratinized mucosa, and this site is the opposite. B describes the first exposure, usually a child with fever, malaise and diffuse painful gingival and mucosal ulceration, not a localised recurrent cluster. D is a genuine unilateral, midline-respecting pattern, but it is reactivation of varicella-zoster after chickenpox, not the recurrent HSV-1 pattern her history describes.

**Common trap:** Sorting oral ulcers by appearance instead of by whether the mucosa is keratinized and bound down.

Source: Neville, Oral and Maxillofacial Pathology

## Question 4

During a recall examination the mother of a healthy 12-year-old boy asks about the HPV vaccine, having read that it prevents throat cancer. He has had no doses and is not immunocompromised. What should the dentist tell her about the schedule recommended for him?

- A. Three doses at 0, 1–2 and 6 months, the schedule used at every age
- B. Two doses, the second given 6 to 12 months after the first, starting now
- C. A single dose now, with a booster only if he later becomes immunocompromised
- D. No doses yet, since vaccination should wait until after his fifteenth birthday

**Answer B:** Two doses, the second given 6 to 12 months after the first, starting now

CDC recommends HPV vaccination routinely at age 11 or 12, which may start at age 9, with catch-up through age 26 and shared clinical decision-making for some adults 27 through 45. The schedule is two doses, the second 6 to 12 months after the first, if the series begins before the 15th birthday, and three doses at 0, 1–2 and 6 months if it begins at 15 through 26 or in an immunocompromised patient. A applies the three-dose schedule universally and over-treats a 12-year-old. C invents a single-dose regimen with a conditional booster. D delays past the routine age and past the band in which the shorter schedule applies. The dental relevance is real: high-risk HPV, especially type 16, is strongly linked to oropharyngeal squamous cell carcinoma.

**Common trap:** Reaching for the three-dose schedule without checking the age at which the series begins.

Source: [CDC/ACIP HPV vaccine recommendations](https://www.cdc.gov/hpv/hcp/vaccination-considerations/index.html)

## Question 5

A 58-year-old woman taking a high-dose inhaled corticosteroid for asthma has white curd-like plaques on the buccal mucosa and dorsum of the tongue. Wiping with gauze removes them and leaves a red, slightly bleeding base. She has no other symptoms. What management does this presentation require?

- A. Topical antifungal therapy together with correction of the predisposing factor
- B. Incisional biopsy of the plaques before any antifungal therapy is started
- C. A systemic azole as first-line therapy, given the extent of the plaques
- D. Reassurance and review, since plaques that wipe off resolve without treatment

**Answer A:** Topical antifungal therapy together with correction of the predisposing factor

White curd-like plaques that wipe off to leave a red, sometimes bleeding base are pseudomembranous candidiasis, and "wipes off" is the single most useful bedside discriminator among oral white lesions. Every presentation of oral candidiasis is opportunistic, so diagnosis is only half the job: the predisposing factor — here inhaled corticosteroid use, and elsewhere antibiotics, xerostomia, poorly controlled diabetes, a denture worn overnight or immune suppression — must be addressed, because antifungal therapy without it produces relapse. B is the pathway for a white lesion that does not rub off, such as chronic hyperplastic candidiasis or oral hairy leukoplakia. C reverses the order of therapy: topical agents are first-line and systemic azoles are reserved for extensive, refractory or immunocompromised cases, carrying meaningful cytochrome P450 interactions. D leaves both infection and cause untreated.

**Common trap:** Treating the infection and never asking what changed about the host.

Source: Neville, Oral and Maxillofacial Pathology

## Question 6

A 63-year-old man with poorly controlled type 1 diabetes, admitted last week in ketoacidosis, attends with a black necrotic area of the hard palate that has enlarged over five days. He has facial pain and nasal congestion on the same side. What is the appropriate next step?

- A. Debride the necrotic palate locally, dress the area, and review in one week
- B. Prescribe a topical antifungal and review once his glycaemic control improves
- C. Refer him at once for systemic antifungal therapy and surgical debridement
- D. Swab the lesion for culture and start an oral antibiotic while awaiting results

**Answer C:** Refer him at once for systemic antifungal therapy and surgical debridement

Mucormycosis is the aggressive deep mycosis: an angioinvasive infection occurring in poorly controlled diabetes, classically with ketoacidosis, and in profound immunosuppression, presenting as palatal necrosis with a black eschar and requiring urgent medical referral, systemic antifungal therapy and surgical debridement. The stem supplies both the host state and the lesion. A treats an angioinvasive systemic infection as a local wound and adds a week of delay while it advances. B applies a topical agent where systemic therapy is required and postpones action pending metabolic control. D covers with an antibacterial that addresses neither the organism nor the necrotic tissue, and spends the waiting period without a referral.

**Common trap:** Managing an angioinvasive fungal infection at the level of the mucosal lesion you can see.

Source: Neville, Oral and Maxillofacial Pathology

## Question 7

A 44-year-old man with a spreading odontogenic infection and systemic involvement is prescribed metronidazole after drainage. A student asks how the drug works and what it will not cover, since the same patient also has a skin infection under review. Which statement is correct?

- A. It inhibits the 50S ribosomal subunit and covers both aerobes and anaerobes
- B. It binds penicillin-binding proteins, so it works best against dividing organisms
- C. It inhibits DNA gyrase, giving broad activity against aerobic Gram-negative rods
- D. It is reduced within anaerobes to damage DNA and lacks aerobic activity

**Answer D:** It is reduced within anaerobes to damage DNA and lacks aerobic activity

Antibiotics are learned by the structure they attack. Metronidazole is reduced inside anaerobic organisms into products that damage DNA, so it is active against anaerobes and protozoa and inactive against aerobes — a spectrum that fits odontogenic infection, where deep, oxygen-poor sites are dominated by obligate anaerobes, and that explains what it will not cover elsewhere. A assigns the macrolide and lincosamide target: the 50S subunit is where azithromycin, clarithromycin, clindamycin and linezolid act, and it also over-claims the spectrum. B describes the β-lactams, which bind penicillin-binding proteins, block peptidoglycan cross-linking and work best against dividing cells. C describes the fluoroquinolones, which inhibit DNA gyrase and topoisomerase IV.

**Common trap:** Assuming a drug used for a serious infection must carry broad aerobic cover.

Source: [Marsh and Martin's Oral Microbiology](https://shop.elsevier.com/books/oral-microbiology/lewis/978-0-443-27894-5)

## Question 8

A 66-year-old woman whose cardiologist confirms she falls in a category for which the American Heart Association recommends prophylaxis needs periodontal surgery. She reports anaphylaxis to penicillin. Her previous dentist's note reads "clindamycin 600 mg one hour before." What should the dentist do with that plan?

- A. Follow it, since clindamycin remains the standard alternative for penicillin allergy
- B. Revise it, because clindamycin was removed as an allergy alternative in 2021
- C. Follow it but move the dose to 30 minutes before the procedure
- D. Omit prophylaxis entirely, since a penicillin allergy removes her eligibility for it

**Answer B:** Revise it, because clindamycin was removed as an allergy alternative in 2021

For an eligible patient the regimen is amoxicillin 2 g as a single oral dose 30–60 minutes before the procedure, and the 2021 American Heart Association statement removed clindamycin as a penicillin-allergy alternative, largely because of Clostridioides difficile risk. A carries forward precisely the recommendation that was withdrawn, which is where older prep material is most reliably wrong. C corrects the timing but leaves the withdrawn drug in place; the timing window belongs to the recommended regimen and cannot rescue this one. D confuses agent selection with eligibility — her cardiac category determines whether prophylaxis is indicated, and the allergy determines only which agent is used. The substitute must be taken from the current AHA statement and wallet card rather than assumed.

**Common trap:** Repeating a prophylaxis regimen from an old note without asking whether the drug is still recommended.

Source: [2021 AHA Scientific Statement + current AHA wallet card](https://www.heart.org/-/media/files/health-topics/infective-endocarditis/infective-endocarditis-wallet-card.pdf)

## Question 9

A healthy 31-year-old man has severe pain and a localised fluctuant buccal swelling beside a necrotic mandibular premolar. He is afebrile, with no trismus, no lymphadenopathy and no malaise, and he asks for antibiotics because they helped a friend. What is the appropriate management?

- A. Definitive dental treatment with drainage and analgesia, without an antibiotic
- B. Amoxicillin now, with definitive treatment deferred until the swelling settles
- C. Amoxicillin with clavulanate now, plus analgesia and review in three days
- D. Analgesia alone, with treatment arranged only if systemic symptoms appear

**Answer A:** Definitive dental treatment with drainage and analgesia, without an antibiotic

The ADA guideline states that antibiotics are not indicated for symptomatic irreversible pulpitis, symptomatic apical periodontitis, or a localised acute apical abscess in an immunocompetent adult without systemic involvement; definitive dental treatment — pulpotomy, pulpectomy, incision and drainage, or extraction — plus analgesia is the management, because an antibiotic neither drains pus nor sterilises necrotic tissue. Systemic involvement means fever, malaise, lymphadenopathy, trismus or spreading swelling, and this stem excludes each of them. B and C both prescribe against the guideline, and B additionally postpones the source control that resolves the problem. D withholds the antibiotic correctly but withholds the definitive treatment too, leaving the collection undrained and the necrotic pulp in place.

**Common trap:** Letting the severity of the pain, rather than systemic involvement, decide whether to prescribe.

Source: [ADA antibiotic guideline for urgent pulpal/periapical pain and swelling](https://www.ada.org/resources/research/science/evidence-based-dental-research/antibiotics-for-dental-pain-and-swelling)

## Question 10

A dental assistant sustains a deep needlestick from a patient known to be HBsAg-positive and HBeAg-positive. She completed a three-dose hepatitis B series years ago, and her anti-HBs measured one month afterwards was 4 mIU/mL. What does that serology mean for her management now?

- A. She is protected, because completing three doses establishes lifelong immunity
- B. She needs no action, because a titre above zero shows she responded
- C. She is susceptible and needs hepatitis B immune globulin plus vaccine promptly
- D. She should be retested in three months before any decision is made

**Answer C:** She is susceptible and needs hepatitis B immune globulin plus vaccine promptly

Protection is defined by antibody rather than by doses given: anti-HBs ≥10 mIU/mL measured 1–2 months after a complete series of three or more doses means seroprotected, with no further periodic testing needed. Her 4 mIU/mL is below that threshold, so she is susceptible, and a susceptible exposed person receives hepatitis B immune globulin 0.06 mL/kg intramuscularly plus vaccine. Urgency matters because HBIG's effectiveness beyond 7 days post-exposure is unknown, and this source is the highest-risk category, with a 22%–31% risk of clinical hepatitis. A and B both treat a completed series or any detectable titre as protection and ignore the defined threshold. D inserts a three-month delay into a decision with a seven-day horizon.

**Common trap:** Reading "completed the vaccine series" as protection instead of checking the anti-HBs threshold.

Source: [CDC hepatitis B guidance for health-care personnel (2013)](https://www.cdc.gov/mmwr/preview/mmwrhtml/rr6210a1.htm)

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Official reference: [JCNDE — Integrated National Board Dental Examination](https://jcnde.ada.org/inbde). Original exam-style questions written for study, never recalled exam content. Independent educational preparation, not clinical advice, and not affiliated with or endorsed by the Joint Commission on National Dental Examinations.