A newborn is found to have a heart rate of 55 bpm with completely dissociated atrial and ventricular activity on ECG. The mother has a history of dry eyes, dry mouth, and positive anti-Ro (SSA) antibodies. What is the most likely cause of the infant's bradycardia?
- A Maternal lithium ingestion during pregnancy
- B Structural defect of the interventricular septum compressing the His bundle
- C Transplacental transfer of maternal autoantibodies causing fibrosis of the fetal AV node ✓
- D Congenital long QT syndrome with 2:1 AV block
Correct answer: C. Transplacental transfer of maternal autoantibodies causing fibrosis of the fetal AV node
Explanation
Congenital complete heart block is strongly associated with maternal anti-Ro and anti-La antibodies, typically in mothers with SLE or Sjogren syndrome. These antibodies cross the placenta and cause fibrous replacement of the fetal AV node, usually detected between 16 and 24 weeks of gestation. Lithium causes Ebstein anomaly, not heart block, and structural septal defects are not the classic association tested here.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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