# INBDE practice questions: FK2 — Physics and chemistry of biology Ten original INBDE practice questions on FK2 — Physics and chemistry of biology, each answered on this page with a rationale and a source. Last updated: 2026-08-10. ## Question 1 A 26-year-old woman in her second trimester has localized pain in tooth #30 and needs a bitewing to assess it. She asks for a lead apron and thyroid collar, saying she was always given them at home. The dentist practises in a state whose radiation regulations do not require shielding. What is the correct course? - A. Defer the radiograph until after delivery and manage the tooth on symptoms alone - B. Provide the apron and collar, since patient reassurance outweighs the technical objection - C. Provide the thyroid collar only, since fetal dose is negligible but thyroid dose is not - D. Take the justified image without shielding, explaining why it is no longer recommended **Answer D:** Take the justified image without shielding, explaining why it is no longer recommended Current ADA and AAOMR recommendations discontinue routine abdominal and thyroid shielding for intraoral, panoramic, cephalometric and cone-beam imaging, for all patients including those who are pregnant, and the AAPD reached the same conclusion for children. Dose to those structures is negligible, shields do not stop internal scatter, and a misplaced shield produces a non-diagnostic image and a retake that adds dose. The image here is justified by a symptomatic tooth, and this state imposes no shielding requirement, so it proceeds with digital receptors and rectangular collimation. Had the state's rules required shielding, that requirement would control. A treats the absence of an apron as a reason to defer care. B substitutes reassurance for the explanation she is owed. C splits a recommendation that names the thyroid explicitly. **Common trap:** Treating pregnancy as an exception to a recommendation that expressly includes pregnant patients. Source: [AAPD — Prescribing dental radiographs for infants, children, adolescents, and individuals with special health care needs](https://www.aapd.org/globalassets/media/policies_guidelines/bp_radiographs.pdf) ## Question 2 A 34-year-old man reports diffuse discomfort in the maxillary left quadrant. No tooth is obviously carious, the periodontium is healthy, and no radiograph has yet been taken. The practice owns a cone-beam unit and the assistant suggests starting with a small-field scan. What should the dentist do? - A. Take a lower-dose examination first, reserving cone-beam imaging for what it alone answers - B. Take the small-field scan now, since its dose is comparable to that of a full-mouth series - C. Take the small-field scan now, since it will settle every possible cause in one exposure - D. Take no images, since diffuse pain with no obvious cause is unlikely to be dental **Answer A:** Take a lower-dose examination first, reserving cone-beam imaging for what it alone answers Every radiation decision has two separate steps, and justification — whether the image will change what you do for this patient — comes before any technical consideration. Cone-beam computed tomography should not be used routinely, nor as the initial modality when a lower-dose examination answers the question, and the ADA and AAOMR patient-selection recommendations are the controlling US document for what to prescribe. B misuses a real figure: adult cone-beam imaging runs from about 19 µSv at a small field of view to over 1,000 µSv at a large one, and proximity to a plain examination does not make it the appropriate first study. C promises a completeness no single image delivers. D abandons a symptomatic patient instead of choosing the lowest-dose examination that answers the question. **Common trap:** Choosing a modality by what the office already owns rather than by what the clinical question requires. Source: ADA Council on Scientific Affairs — Optimizing radiation safety in dentistry ## Question 3 A 45-year-old man who reads widely about health asks, before four bitewings are taken, whether dental x-rays could burn his skin or cause a cataract, and whether there is a dose below which he can consider himself entirely safe. How should the dentist answer? - A. Both effects are possible at dental doses, so images should be limited to emergencies - B. Those injuries need doses far above dental exposures; random risk has no assumed threshold - C. Neither injury nor random risk applies below one millisievert, a recognized safe threshold - D. Both injury and random risk rise in proportion to dose, with no threshold for either **Answer B:** Those injuries need doses far above dental exposures; random risk has no assumed threshold Biological effects split in two. Tissue reactions, the deterministic effects — skin injury, cataract, cell death — require an acute dose above a threshold generally cited above 100 mGy, far above dental exposures; four bitewings are 3.4 to 5.0 µSv, under a day of the 3.1 mSv-a-year natural background the same comparison cites. Stochastic effects are random: probability rises with dose and no dose is assumed entirely risk-free, which is why no exposure can be called absolutely safe. A inflates a dental exposure into the threshold range. C invents a safe threshold and applies it to the very category defined by not having one. D collapses the distinction, giving the threshold effects a no-threshold behaviour they do not have. **Common trap:** Applying a single dose-response model to both categories of radiation effect. Source: ADA Council on Scientific Affairs — Optimizing radiation safety in dentistry ## Question 4 A 27-year-old man is being examined for early proximal caries. His previous bitewings, exposed at 90 kVp, show many shades of grey, and the operator found the enamel–dentine boundary hard to follow. The dentist wants a higher-contrast image on the retake. Which change achieves it? - A. Raise the kVp, since greater penetration produces a shorter contrast scale - B. Raise the mA, since more photons produce a shorter contrast scale - C. Lower the kVp, since less penetration produces a shorter contrast scale - D. Lower the mAs, since fewer photons produce a shorter contrast scale **Answer C:** Lower the kVp, since less penetration produces a shorter contrast scale A higher kVp beam is more penetrating, so more photons reach the receptor from behind dense structures; that raises exposure and lowers contrast, producing the long-scale, many-greys image described in the stem. Lowering the kVp does the reverse and gives a short-scale image of mostly blacks and whites, which is the easier image on which to spot a proximal lesion; long scale is better reserved for judging bone and soft-tissue detail. A states the correct mechanism attached to the wrong direction and would lengthen the scale further. B and D both change the number of photons rather than their energy, and photon quantity governs density only — neither raising nor lowering mAs alters the contrast scale at all, though either will change how dark the image looks. **Common trap:** Assuming any exposure change that alters the image's appearance must have altered its contrast. Source: White and Pharoah's Oral Radiology: Principles and Interpretation ## Question 5 A 36-year-old man needs an 18-image full-mouth series. The office unit carries a round 7 cm collimator, its filtration meets the federal requirement, and D-speed film is not in use. The dentist wants the single change that most reduces this patient's dose without losing information. Which should she make? - A. Reduce the round field to 6 cm, the diameter the federal standard requires - B. Add filtration above the required half-value layer for this operating potential - C. Raise the kVp to 90 so that fewer photons are absorbed in tissue - D. Fit rectangular collimation matched to the receptor, cutting dose over 40% **Answer D:** Fit rectangular collimation matched to the receptor, cutting dose over 40% Rectangular collimation is the largest technique lever available here: the ADA reports it reduces patient dose by more than 40%, and its own dose table shows the same adult 18-image full-mouth survey at about 34.9 µSv with rectangular collimation against 170.7 µSv with round — roughly five times the dose for identical diagnostic information. A misreads the federal beam-limitation rule, under which 7 cm is the permitted ceiling when the source-to-skin distance is 18 cm or more and 6 cm applies only when that distance is less; it is a legal ceiling, not an optimisation target. B adds filtration to a unit that already complies, with no comparable benefit. C changes the contrast scale on a caries survey and is not the dose lever the evidence identifies. **Common trap:** Treating the federal collimation ceiling as the recommended standard rather than the legal floor for compliance. Source: [FDA performance standards for diagnostic x-ray systems, 21 CFR 1020.30 and 1020.31](https://www.govinfo.gov/content/pkg/CFR-2024-title21-vol8/xml/CFR-2024-title21-vol8-sec1020-30.xml) ## Question 6 A 29-year-old woman has had four bitewings, two periapicals and a panoramic image in the past eight months. She now presents with acute pain at a different tooth that requires a periapical to diagnose. The assistant says she is "close to her annual limit" and should wait. How should the dentist respond? - A. She is limited to 1 mSv a year, the figure set for the public - B. She is limited to 50 mSv a year, the occupational adult figure - C. She is limited to 5 mSv, the total figure across an entire declared pregnancy - D. No numeric limit binds her; each exposure must be justified and optimised **Answer D:** No numeric limit binds her; each exposure must be justified and optimised There is no numeric regulatory limit on diagnostic patient exposure. Patient imaging is governed by two ideas instead: justification, meaning the image must be capable of changing management, and optimisation, meaning it is made at the lowest dose consistent with the diagnostic task. A symptomatic tooth that cannot be diagnosed without an image satisfies justification regardless of how many images preceded it. A borrows the public limit of 0.1 rem (1 mSv) per year total effective dose equivalent. B borrows the occupational adult annual limit of 5 rem (0.05 Sv, that is 50 mSv). C borrows the 0.5 rem (5 mSv) limit that applies to the embryo or fetus of a declared pregnant worker across the whole pregnancy. Each is a real figure attached to the wrong person. **Common trap:** Transplanting an occupational or public dose limit onto a patient, where no such cap exists. Source: [NRC Standards for Protection Against Radiation, 10 CFR Part 20](https://www.govinfo.gov/content/pkg/CFR-2024-title10-vol1/xml/CFR-2024-title10-vol1-sec20-1201.xml) ## Question 7 A colleague trained abroad tells a new graduate that ALARA has been retired in the United States and replaced by ALADA, and that a third acronym, ALADAIP, is now the operative principle for paediatric imaging. The graduate asks which account is right. Which statement matches current US recommendations? - A. ALARA was retired in 2024 and ALADA has replaced it entirely - B. ALARA remains the overarching principle, with ALADA endorsed alongside it - C. ALADAIP is the principle current ADA documents direct clinicians to follow - D. Neither applies to dental imaging, since dental effective doses are negligible **Answer B:** ALARA remains the overarching principle, with ALADA endorsed alongside it The ADA's 2024 expert panel calls ALARA "firmly entrenched as an overarching principle," and the 2026 ADA/AAOMR patient-selection statement directs clinicians to follow ALARA and ALADA — as low as diagnostically acceptable, the refinement that arrived with cone-beam imaging. The AAPD likewise still teaches ALARA. The two principles are complementary, not sequential. A asserts a retirement that no current US document records, and the body that would have had to retire it reaffirmed it instead. C names an acronym circulating in the paediatric cone-beam research literature that appears in no current ADA or ADA/AAOMR recommendation. D misuses a true premise: dental effective doses are small, but stochastic risk is modelled without a threshold, so optimisation still applies to every exposure. **Common trap:** Assuming a newer acronym must have replaced the older principle rather than refined it. Source: [AAPD — Prescribing dental radiographs for infants, children, adolescents, and individuals with special health care needs](https://www.aapd.org/globalassets/media/policies_guidelines/bp_radiographs.pdf) ## Question 8 A 30-year-old woman who is 14 weeks pregnant has acute pain from a mandibular molar, and a periapical image is needed to decide treatment. She asks for a lead apron and thyroid collar. The state's radiation rules contain no shielding requirement. What should the dentist do? - A. Defer all imaging until after delivery, since fetal exposure has no safe threshold - B. Apply the apron and collar, since pregnancy is carved out of the recommendation - C. Take the image without routine shielding, explaining the current guidance to her - D. Refer her to a physician to authorise the exposure before any image is taken **Answer C:** Take the image without routine shielding, explaining the current guidance to her The AAOMR and the ADA both recommend that patient gonadal, fetal, abdominal and thyroid shielding be discontinued as routine practice for intraoral, panoramic, cephalometric and cone-beam imaging, in adults and children alike, and the AAPD endorses the same position. Pregnancy is not carved out: the recommendation rests on the finding that dentomaxillofacial imaging delivers negligible exposure to those tissues, and that the out-of-field dose which does reach them is internal scatter generated inside the patient, which an external apron cannot attenuate. B states an exception the documents do not contain. A withholds diagnosis from a patient in pain over a risk the evidence does not support, and pregnancy does not contraindicate necessary dental radiography. D invents a medical authorisation step for a dental diagnostic image. Where state rules still mandate shielding, state law governs. **Common trap:** Treating pregnancy as an automatic exception to a recommendation written to include it. Source: [AAPD — Prescribing dental radiographs for infants, children, adolescents, and individuals with special health care needs](https://www.aapd.org/globalassets/media/policies_guidelines/bp_radiographs.pdf) ## Question 9 A 30-year-old man is to receive 4% articaine with 1:100,000 epinephrine from cartridges labeled 1.7 mL. A student asks how many milligrams of articaine each cartridge contains, having memorized that "a dental cartridge is 1.8 mL." What is the correct figure for this box? - A. 34 mg, because a 4% solution contains 20 milligrams per millilitre - B. 68 mg, because 4% is 40 mg/mL multiplied by 1.7 mL - C. 72 mg, because every US dental cartridge holds 1.8 millilitres of solution - D. 40 mg, because the percentage already states milligrams per whole cartridge **Answer B:** 68 mg, because 4% is 40 mg/mL multiplied by 1.7 mL A percentage is grams per 100 mL, so 4% is 40 mg/mL; multiplied by the 1.7 mL printed on the carton, each cartridge contains 68 mg of articaine, exactly as the label states. A runs 2% arithmetic — 20 mg/mL — producing the 34 mg figure that belongs to 2% lidocaine or 2% mepivacaine in the same 1.7 mL cartridge. C imports prilocaine's convention: Citanest is dispensed in 1.8 mL cartridges of 72 mg, which is precisely why a memorized "every cartridge is 1.8 mL" rule fails on a US lidocaine, articaine or mepivacaine box. D treats the percentage as a mass per cartridge and skips the volume step entirely. On an exam item, use the volume the stem supplies; in the operatory, read the carton. **Common trap:** Assuming a single cartridge volume applies to every US dental anesthetic. Source: [Septocaine (articaine HCl 4% and epinephrine 1:100,000 or 1:200,000), FDA-approved labeling](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=0eba0bd2-ccb6-4f7e-8d0c-e4b4e6cdce03) ## Question 10 A 33-year-old woman has a swollen, tender mandibular molar with acute apical infection. A buccal infiltration placed directly over the swelling produces almost no numbness, and nothing about the technique appeared wrong. What is the chemical reason, and the appropriate response? - A. The infection destroyed the sodium channels; give a different amide instead - B. Purulent tissue dilutes the solution; deposit twice the volume there - C. Bacteria metabolize the anesthetic; add a vasoconstrictor to slow breakdown - D. Low tissue pH blocks free-base diffusion; inject away from the infection **Answer D:** Low tissue pH blocks free-base diffusion; inject away from the infection A local anesthetic must cross the nerve membrane in its uncharged free-base form, and infection lowers the pH of the extracellular tissue, inhibiting diffusion of the active free base across the neural membrane; onset is prolonged or the block simply fails. The practical answer is to inject away from the infected area — a block proximal to the infection rather than an infiltration into it. A invents destruction of sodium channels and changes drug without changing the chemistry that defeated the first injection. B repeats the same failed deposit at double volume, which walks the patient toward the maximum recommended dose while facing the same pH. C invents bacterial metabolism of the anesthetic; the vasoconstrictor slows systemic absorption and controls bleeding. Buffering has been studied with mixed results, so it settles nothing here. **Common trap:** Adding volume to an acidic field instead of moving the injection out of it. Source: [AAPD local-anesthesia best practice](https://www.aapd.org/globalassets/media/policies_guidelines/bp_localanesthesia25.pdf) ## Next step [Take the free INBDE practice test](https://dentovio.com/inbde/free-practice-test) 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.