A 19-year-old college student presents to the student health center with a 2-day history of sore throat, fever, and fatigue. She reports difficulty swallowing and swollen glands in her neck. Vital signs show a temperature of 38.5°C (101.3°F). Physical examination reveals exudative tonsillitis and tender cervical lymphadenopathy. A rapid Strep test is negative, but a throat culture grows gram-positive cocci in chains which are beta-hemolytic on blood agar and bacitracin-sensitive.
Which of the following is the most likely causative organism?
The correct answer is:
E) Streptococcus pyogenes
The presentation of sore throat, fever, exudative tonsillitis, and cervical lymphadenopathy, with throat culture showing gram-positive cocci, beta-hemolytic, and bacitracin-sensitive, is consistent with Streptococcus pyogenes (group A Streptococcus) pharyngitis. This is the most common bacterial cause of pharyngitis and is treated with penicillin or amoxicillin to prevent complications like rheumatic fever.
Answer choice A: Arcanobacterium haemolyticum, is incorrect. Arcanobacterium haemolyticum causes pharyngitis with rash in young adults and is beta-hemolytic, but is a gram-positive bacillus (not coccus) and is bacitracin-insensitive.
Answer choice B: Corynebacterium diphtheriae, is incorrect. Corynebacterium diphtheriae is a gram-positive bacillus forming a pseudomembrane in diphtheria. It is not a coccus or beta-hemolytic and is bacitracin-insensitive.
Answer choice C: Streptococcus agalactiae, is incorrect. Streptococcus agalactiae (group B Streptococcus) is beta-hemolytic but bacitracin-resistant and causes neonatal sepsis/meningitis, not pharyngitis.
Answer choice D: Streptococcus pneumoniae, is incorrect. Streptococcus pneumoniae is alpha-hemolytic and optochin-sensitive, causing pneumonia or otitis, not pharyngitis.
Key Learning Point
Streptococcus pyogenes causes bacterial pharyngitis, identified by beta-hemolytic, bacitracin-sensitive gram-positive cocci. A negative rapid test should be followed by culture confirmation.
