Examkin

Question

Difficulty: Very hardPediatric Infectious Diseases

A 4-week-old female infant is brought to the pediatric clinic by her mother due to poor feeding, nasal congestion, and a rash. The infant was born at 39 weeks gestation via uncomplicated vaginal delivery to a 22-year-old primigravida. The mother began prenatal care at 28 weeks gestation, at which time a rapid plasma reagin (RPR) test was nonreactive. At delivery, a repeat maternal RPR was ordered and reported as nonreactive. The mother reports no history of sexually transmitted infections. On examination, the infant is in the 5th percentile for weight. She has copious, blood-tinged nasal discharge. A diffuse, erythematous, maculopapular rash with areas of superficial desquamation is present on the trunk, palms, and soles. The infant cries vigorously when her right arm is palpated and maintains the arm in a rigid, extended position. Abdominal examination reveals the liver edge 4 cm below the right costal margin. Given the high clinical suspicion, a serum RPR is obtained from the infant and returns reactive at a titer of 1:1024. Confirmatory fluorescent treponemal antibody absorption (FTA-ABS) testing is positive. Which of the following best explains the discrepant maternal and infant serologic findings at the time of delivery?

  1. High concentration of maternal non-treponemal antibodies preventing visible agglutinationAnswer
  2. B
    Cross-reacting antibodies from an infant viral lower respiratory infection causing a false-positive result
  3. C
    Maternal intrapartum broad-spectrum antibiotic administration ablating the serologic response
  4. D
    Systemic lipid alterations related to a benign transitional neonatal exanthem
  5. E
    Humoral immune suppression secondary to inappropriate maternal live-attenuated vaccination

Answer

High concentration of maternal non-treponemal antibodies preventing visible agglutination
The infant's clinical presentation (snuffles, palmar/plantar desquamating rash, hepatomegaly, pseudoparalysis of Parrot) and highly reactive serology definitively confirm congenital syphilis. The mother's false-negative RPR at delivery is due to the prozone phenomenon. The RPR is a flocculation test that relies on an optimal ratio of antigen to antibody to form a visible lattice. When a patient has an exceedingly high concentration of non-treponemal antibodies (often seen in secondary syphilis or late-acquired pregnancy syphilis), the antibodies overwhelm the available antigen, preventing cross-linking and lattice formation. The laboratory must dilute the serum to reach the 'zone of equivalence' and observe the positive reaction.

Step-by-Step Solution

1
Identify the infant's clinical syndrome.
The infant has snuffles, a palmar/plantar desquamating rash, hepatomegaly, and pseudoparalysis of Parrot.
These findings are highly specific for symptomatic congenital syphilis.
2
Analyze the laboratory findings.
The infant has a confirmed reactive RPR (1:1024) and positive FTA-ABS, proving active infection.
This establishes that the infant's disease is genuine, meaning the mother's negative test at delivery was a false negative.
3
Determine the pathophysiological mechanism for the false-negative maternal test.
The prozone phenomenon caused the nonreactive maternal RPR.
In cases of massive antibody excess (often seen in secondary syphilis or late pregnancy acquisition), the antibodies fail to form the antigen-antibody lattice required for visible flocculation.

Key Concept

The prozone phenomenon is a false-negative RPR result caused by disproportionately high non-treponemal antibody titers that prevent the formation of the antigen-antibody lattice required for agglutination.
Estimated Time:1m 30s
Rate this question

Topics

Pediatric Infectious Diseases

More questions from this topic