Doxycycline – chemistry and use MCQs With Answer

Doxycycline – chemistry and use MCQs With Answer
Doxycycline is a semisynthetic tetracycline antibiotic characterized by a tetracyclic naphthacene core and multiple ionizable functional groups. Key concepts for B.Pharm students include its mechanism of action (reversible binding to the 30S ribosomal subunit), high oral bioavailability, long half-life, chelation with divalent/trivalent cations, biliary/fecal elimination, and common adverse effects such as photosensitivity and tooth discoloration. Understanding formulations (hyclate vs monohydrate), analytical methods (HPLC, microbiological assays), resistance mechanisms (efflux pumps, ribosomal protection), and clinical uses (atypicals, rickettsiae, malaria prophylaxis) is essential. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. Which primary mechanism of action best describes doxycycline?

• Inhibits DNA gyrase preventing DNA replication
• Interferes with peptidoglycan cross-linking
• Binds to the 30S ribosomal subunit blocking protein synthesis
• Disrupts bacterial cell membrane integrity

Correct Answer: Binds to the 30S ribosomal subunit blocking protein synthesis

Q2. Doxycycline belongs to which chemical class?

• Macrolide antibiotic
• Tetracycline derivative
• Fluoroquinolone class
• Aminoglycoside family

Correct Answer: Tetracycline derivative

Q3. Which description best defines the core structure of doxycycline?

• Beta-lactam bicyclic ring
• Tetracyclic naphthacene ring system
• Macrocyclic lactone ring
• Benzimidazole heterocycle

Correct Answer: Tetracyclic naphthacene ring system

Q4. Which interaction significantly reduces oral doxycycline absorption?

• Concurrent proton pump inhibitor use
• Co-administration with divalent/trivalent cations (Ca2+, Mg2+, Fe3+)
• Administration with high-fat meals only
• Taking with vitamin C supplements

Correct Answer: Co-administration with divalent/trivalent cations (Ca2+, Mg2+, Fe3+)

Q5. What is the approximate oral bioavailability of doxycycline?

• Less than 10%
• About 30–50%
• High, approximately 90–100%
• Variable and negligible

Correct Answer: High, approximately 90–100%

Q6. What is the typical elimination half-life of doxycycline in adults?

• 1–3 hours
• 6–8 hours
• Approximately 16–22 hours
• Over 72 hours

Correct Answer: Approximately 16–22 hours

Q7. Which statement best describes doxycycline excretion?

• Primarily renal excretion with high urinary concentrations
• Mostly biliary/fecal excretion with minimal renal clearance
• Eliminated unchanged exclusively via lungs
• Rapid metabolism to inactive metabolites excreted in sweat

Correct Answer: Mostly biliary/fecal excretion with minimal renal clearance

Q8. What is the recommendation regarding doxycycline use in pregnancy and young children?

• Preferred antibiotic in pregnancy and children under 8
• Safe in first trimester but contraindicated later
• Contraindicated in pregnancy and children under 8 years
• Only topical use is contraindicated

Correct Answer: Contraindicated in pregnancy and children under 8 years

Q9. Which adverse effect is classically associated with doxycycline and other tetracyclines?

• Renal papillary necrosis as the most common effect
• Photosensitivity leading to exaggerated sunburn
• Severe neutropenia in most patients
• Significant hypoglycemia episodes

Correct Answer: Photosensitivity leading to exaggerated sunburn

Q10. Which clinical problem can occur if doxycycline tablets are swallowed without enough water or immediately before lying down?

• Aspiration pneumonia
• Esophageal irritation or ulceration
• Cardiac arrhythmia
• Acute hepatic failure

Correct Answer: Esophageal irritation or ulceration

Q11. Which resistance mechanisms are commonly seen against doxycycline?

• Beta-lactamase production
• Efflux pumps and ribosomal protection proteins
• Mutations in DNA gyrase only
• Overproduction of dihydrofolate reductase

Correct Answer: Efflux pumps and ribosomal protection proteins

Q12. Which organisms are typically susceptible to doxycycline?

• Only anaerobic Gram-positive cocci
• Atypical organisms (Mycoplasma, Chlamydia), rickettsiae, and some Gram-positive/Gram-negative bacteria
• Polymicrobial strict aerobes exclusively
• Only Pseudomonas aeruginosa and Enterococcus faecalis

Correct Answer: Atypical organisms (Mycoplasma, Chlamydia), rickettsiae, and some Gram-positive/Gram-negative bacteria

Q13. Which of the following salt forms of doxycycline are commonly used in pharmaceutical formulations?

• Doxycycline sulfate and doxycycline nitrate
• Doxycycline hyclate and doxycycline monohydrate
• Doxycycline acetate and doxycycline phosphate
• Doxycycline chloride and doxycycline bromide

Correct Answer: Doxycycline hyclate and doxycycline monohydrate

Q14. Regarding chemical stability, doxycycline is best stored how?

• Exposed to light at room temperature for potency
• Protected from light and moisture in a tight container
• Frozen to ensure long-term stability
• Stored in alkaline aqueous solution

Correct Answer: Protected from light and moisture in a tight container

Q15. Which acid–base property characterizes doxycycline?

• Strong monobasic acid with one pKa
• Non-ionizable neutral compound
• Amphoteric molecule with multiple ionizable groups (multiple pKa values)
• Permanent cationic quaternary ammonium compound

Correct Answer: Amphoteric molecule with multiple ionizable groups (multiple pKa values)

Q16. Pharmacodynamically, doxycycline is generally considered to be:

• Bactericidal against most pathogens
• Bacteriostatic by inhibiting protein synthesis
• Only fungistatic, not antibacterial
• Virucidal for enveloped viruses

Correct Answer: Bacteriostatic by inhibiting protein synthesis

Q17. Which co-administered agent is most likely to reduce doxycycline absorption?

• Oral antacid containing calcium or magnesium
• Oral insulin
• Topical corticosteroid cream
• Subcutaneous heparin

Correct Answer: Oral antacid containing calcium or magnesium

Q18. Which non-infectious prophylactic use is doxycycline commonly prescribed for?

• Prophylaxis of deep vein thrombosis
• Malaria prophylaxis in travelers to endemic regions
• Daily prevention of migraine headaches
• Long-term osteoporosis prevention

Correct Answer: Malaria prophylaxis in travelers to endemic regions

Q19. Why is doxycycline effective in acne therapy beyond antibacterial action?

• It increases sebaceous gland activity
• It has anti-inflammatory properties that reduce inflammatory lesions
• It permanently destroys skin melanocytes
• It acts as a keratolytic agent

Correct Answer: It has anti-inflammatory properties that reduce inflammatory lesions

Q20. Which analytical technique is commonly used for quantifying doxycycline in formulations and plasma?

• Thin-layer chromatography only
• High-performance liquid chromatography (HPLC)
• Colorimetric nitrate assay
• Gram staining

Correct Answer: High-performance liquid chromatography (HPLC)

Q21. Compared with older tetracyclines, doxycycline is more stable under which condition?

• Alkaline hydrolysis at high pH
• Acidic gastric pH, showing greater acid stability
• Extremely high temperatures above 200°C
• Strong oxidizing environments only

Correct Answer: Acidic gastric pH, showing greater acid stability

Q22. What long-term developmental effect is associated with tetracycline exposure in children?

• Permanent white hair discoloration
• Permanent discoloration of developing teeth and enamel hypoplasia
• Enhanced linear growth and bone density
• Irreversible improvement in vision

Correct Answer: Permanent discoloration of developing teeth and enamel hypoplasia

Q23. Typical adult dosing frequency for doxycycline in many indications is:

• Every 4 hours (six times daily)
• Once or twice daily (e.g., 100 mg once or twice daily)
• Once weekly only
• Continuous IV infusion without bolus

Correct Answer: Once or twice daily (e.g., 100 mg once or twice daily)

Q24. Which resistance gene encodes a ribosomal protection protein that can confer tetracycline resistance?

• blaTEM
• tet(M)
• mecA
• vanA

Correct Answer: tet(M)

Q25. Which microbiological assay is suitable for potency testing of doxycycline formulations in addition to HPLC?

• Agar well diffusion/microbiological assay using a sensitive indicator strain
• Gram staining of tablet powder
• ELISA for penicillin-binding proteins
• Western blot for tetracycline binding

Correct Answer: Agar well diffusion/microbiological assay using a sensitive indicator strain

Q26. Historically, how were tetracyclines classified by FDA for use in pregnancy?

• FDA category A (safe in pregnancy)
• FDA category B (no evidence of risk)
• FDA category D (evidence of fetal risk; contraindicated)
• FDA category X (teratogenic in all cases)

Correct Answer: FDA category D (evidence of fetal risk; contraindicated)

Q27. At the ribosomal level, doxycycline primarily prevents which step of protein synthesis?

• Peptidyl transferase activity at the 50S subunit
• Aminoacyl-tRNA binding to the A-site of the 30S subunit
• Initiation complex formation by blocking IF3
• Termination by inhibiting release factors

Correct Answer: Aminoacyl-tRNA binding to the A-site of the 30S subunit

Q28. Which common gastrointestinal adverse effect should patients be counseled about when taking doxycycline?

• Constipation with hemorrhoids
• Nausea, vomiting, and abdominal discomfort
• Fasting hypoglycemia
• Marked appetite increase leading to weight gain

Correct Answer: Nausea, vomiting, and abdominal discomfort

Q29. Which doxycycline formulation is generally considered to cause less gastrointestinal irritation?

• Doxycycline hyclate causes less GI irritation than monohydrate
• Doxycycline monohydrate causes less GI irritation than hyclate
• Both forms cause identical GI effects in all patients
• Only IV formulations cause less GI irritation; oral forms are equally irritating

Correct Answer: Doxycycline monohydrate causes less GI irritation than hyclate

Q30. Which dietary habit can reduce doxycycline oral absorption and should be avoided near dosing?

• Drinking plain water with the tablet
• Taking with dairy products (milk, yogurt) due to calcium binding
• Consuming sugar-free gum immediately after dose
• Light exercise within 10 minutes of dosing

Correct Answer: Taking with dairy products (milk, yogurt) due to calcium binding

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com