A teenage patient with pneumonia presents with Gram-positive rods that are CAMP-positive and catalase-negative after culture. Which organism is most likely?

• A. Listeria monocytogenes
• B. Streptococcus agalactiae
• C. Arcanobacterium pyogenes
• D. Streptobacillus moniliformis

Answer: (C)

The key idea is using morphology and two simple tests to narrow down a Gram-positive organism. A Gram-positive rod that is catalase-negative helps rule out Listeria (which is catalase-positive) and points away from many cocci like Streptococcus agalactiae. The CAMP test adds another clue: a CAMP-positive result means the organism produces a CAMP factor that enhances hemolysis when grown near Staphylococcus aureus. Among the options, the slender Gram-positive rod that is catalase-negative and CAMP-positive fits Arcanobacterium pyogenes. Streptobacillus moniliformis is Gram-negative, so it doesn’t fit. Streptococcus agalactiae would be Gram-positive cocci, not rods, so it also doesn’t fit. Arcanobacterium pyogenes is the best match for a CAMP-positive, catalase-negative Gram-positive rod in this context.