A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A 25-year-old G1P0 woman at 33 weeks gestation presents to the obstetrician for an episode of postcoital spotting. The patient’s pregnancy is complicated by diet-controlled gestational diabetes. She has no other medical conditions. She takes prenatal vitamins. She denies tobacco, alcohol, or recreational drug use. She is currently sexually active with her boyfriend of 1 year, but prior to her current relationship, she states she had multiple male partners. On physical examination, no vaginal bleeding is appreciated. The cervix is closed, and there is no leakage of fluid or contractions. Fetal movement is normal. Fundal height is 33 cm. Fetal pulse is 138/min. The patient’s temperature is 37.0 °C (98.6°F), blood pressure is 112/75 mm Hg, and pulse is 76/min. A urine dipstick is negative for glucose and protein. Chlamydia trachomatis nucleic acid amplification testing is positive. Which of the following is the mechanism behind the first-line treatment for this patient’s condition?
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A
Disrupts peptidoglycan cross-linkingIncorrect. Disrupts peptidoglycan cross-linking = beta-lactams/glycopeptides; chlamydia lacks classical peptidoglycan and beta-lactams are not used.
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B
Inhibits DNA gyraseIncorrect. Inhibits DNA gyrase = fluoroquinolones, which are avoided in pregnancy due to cartilage toxicity in animal studies.
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C
Inhibits the 50S ribosome subunitCorrect. Azithromycin (50S ribosomal inhibitor) is first-line for chlamydia in pregnancy — single 1 g oral dose; safe and effective.
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D
Inhibits transpeptidase and cell wall synthesisIncorrect. Inhibits transpeptidase = beta-lactams (penicillins/cephalosporins); they don't reliably treat intracellular chlamydia.
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E
Inhibits folate synthesisIncorrect. Sulfonamide/trimethoprim folate inhibitors are contraindicated in late pregnancy due to kernicterus and neural tube risks and have no role in chlamydia treatment.
↑ Tap an answer to reveal the reasoning
Answer: C. This pregnant patient at 33 weeks tests positive for Chlamydia trachomatis. Untreated maternal chlamydia can transmit to the neonate during vaginal delivery, causing neonatal conjunctivitis (5–14 days postpartum) or chlamydial pneumonia (1–3 months postpartum). All pregnant women with chlamydia should be treated.
In pregnancy, doxycycline (the usual first-line agent for non-pregnant adults) is contraindicated due to fetal teeth and bone effects, and fluoroquinolones are avoided. The first-line treatment in pregnancy is azithromycin 1 g orally as a single dose. Azithromycin is a macrolide that inhibits the 50S ribosomal subunit, blocking translocation and protein synthesis.
The other mechanisms describe different antibiotic classes: disrupting peptidoglycan cross-linking = beta-lactams/penicillins (but these are inactive against intracellular chlamydia which lacks classical peptidoglycan). Inhibiting DNA gyrase = fluoroquinolones (avoided in pregnancy). Inhibiting transpeptidase and cell wall synthesis = beta-lactams again. The unique mechanism here is 50S ribosomal inhibition by azithromycin — the pregnancy-safe drug of choice for chlamydia.
**Why each option:**
**A.** Disrupts peptidoglycan cross-linking = beta-lactams/glycopeptides; chlamydia lacks classical peptidoglycan and beta-lactams are not used.
**B.** Inhibits DNA gyrase = fluoroquinolones, which are avoided in pregnancy due to cartilage toxicity in animal studies.
**C.** Correct. Azithromycin (50S ribosomal inhibitor) is first-line for chlamydia in pregnancy — single 1 g oral dose; safe and effective.
**D.** Inhibits transpeptidase = beta-lactams (penicillins/cephalosporins); they don't reliably treat intracellular chlamydia.