An 8-year-old boy develops periorbital oedema and haematuria two weeks after treated impetigo. Antistreptolysin O titre is reported as low or normal. What is the most likely explanation?
- A Streptolysin O is inhibited by lipids present in the skin, so little antibody is generated ✓
- B Antibiotic therapy given for impetigo abolished the antibody response
- C The renal lesion was caused by Staphylococcus aureus rather than Streptococcus pyogenes
- D Antistreptolysin O antibodies appear only after the second episode of infection
Correct answer: A. Streptolysin O is inhibited by lipids present in the skin, so little antibody is generated
Explanation
Streptolysin O is an oxygen-labile haemolysin whose activity is neutralised by cholesterol. Skin lipids rich in cholesterol inhibit the toxin locally during impetigo, so the antigenic stimulus is poor and the ASO titre rises little or not at all. In contrast, ASO titres rise reliably after pharyngitis. Antibiotic therapy may blunt titres somewhat but does not abolish them, and the nephritis here followed impetigo, making option C less consistent than the classic lipid-inhibition explanation.
Reference: Jawetz, Melnick and Adelberg's Medical Microbiology, 28th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.