A 14-year-old girl comes to the physician with dark urine and facial puffiness. About 4 weeks before presentation, she had pustular skin lesions that broke down over a few days to form thick scabs in the lower extremities. Microscopic examination of the urine sediment shows red blood cell casts. The organism responsible for this patient's symptoms would most likely demonstrate which of the following?

Question

A 14-year-old girl comes to the physician with dark urine and facial puffiness.  About 4 weeks before presentation, she had pustular skin lesions that broke down over a few days to form thick scabs in the lower extremities.  Microscopic examination of the urine sediment shows red blood cell casts.  The organism responsible for this patient's symptoms would most likely demonstrate which of the following?

Quizplus · Accepted Answer

11ec160b_3851_aa00_9249_d34e2133d917_MD0001_00 The patient had impetigo, a skin infection caused by Staphylococcus aureus and less commonly by Streptococcus pyogenes (Group A streptococci [GAS]). Her facial puffiness (likely due to sodium and water retention and possibly proteinuria) and hematuria (likely due to nephritis) suggest post-streptococcal glomerulonephritis. Staphylococcal infection is not complicated by glomerulonephritis, so S pyogenes is the most likely causative organism of her symptoms. Nonsuppurative complications of GAS infections include acute rheumatic fever (ARF) and glomerulonephritis, thought to be due to the formation of different anti-streptococcal antibodies that cross-react with heart tissue or result in immune complex deposition in the kidney. Glomerulonephritis is associated with prior streptococcal pharyngeal or skin infection (eg, cellulitis, impetigo, erysipelas); ARF is associated with prior streptococcal pharyngitis but not skin infection. Previous streptococcal infection is evidenced by elevated streptococcal antibodies (anti-streptolysin, anti-DNaseB, and anti-hyaluronidase). S pyogenes are catalase-negative, β-hemolytic, gram-positive cocci in chains. S pyogenes is unique among β-hemolytic streptococci as it is susceptible to bacitracin. Because the bacitracin test is not very specific for S pyogenes, it has been replaced in many laboratories by the pyrrolidonyl arylamidase (PYR) test; S pyogenes is PYR-positive. (Choices A and D) Streptococcus pneumoniae is bile-soluble and optochin-susceptible. These tests are used to differentiate S pneumoniae from other α-hemolytic streptococci such as those in the viridans group. (Choice B) Streptococci are catalase-negative, staphylococci are catalase-positive. (Choice C) Enterococci, not GAS, can grow in broth containing hypertonic (6.5%) saline. Both S pyogenes and enterococci are PYR-positive. Educational objective: Glomerulonephritis is associated with prior streptococcal pharyngeal or skin infection; acute rheumatic fever is associated with prior streptococcal pharyngitis but not skin infection. Streptococci are catalase-negative, staphylococci are catalase-positive. Streptococcus pyogenes (Group A Streptococcus) is β-hemolytic, bacitracin-susceptible, and pyrrolidonyl arylamidase-positive. Streptococcus pneumoniae is bile-soluble and optochin-susceptible.

A 14-Year-Old Girl Comes to the Physician with Dark Urine