An 8 year old boy is brought to the emergency room with a 3-day history of fever to 102 degrees F and abdominal pain. He also complains of pain in his right knee and right elbow. He was seen four weeks ago because of a sore throat and a rash. A throat culture performed at that time grew gram-positive cocci in chains. Amoxicillin was prescribed, but the boy’s mother did not fill the prescription. On physical exam his temp is 101.7F, HR 96, and BP 100/60. Cardiac exam reveals a pansystolic blowing murmur heard best at the apex. His right elbow is tender on extension and flexion with mild swelling. Laboratory tests reveal a positive C-reactive protein, an ESR of 40 and a WBC of 22,000 with a left shift. ECG shows a prolonged PR interval. Which of the following tests would be positive for the microorganism responsible for this patient’s illness?
An 8 year old boy is brought to the emergency room with a 3-day history of fever to 102 degrees F and abdominal pain. He also complains of pain in his right knee and right elbow. He was seen four weeks ago because of a sore throat and a rash. A throat culture performed at that time grew gram-positive cocci in chains. Amoxicillin was prescribed, but the boy’s mother did not fill the prescription. On physical exam his temp is 101.7F, HR 96, and BP 100/60. Cardiac exam reveals a pansystolic blowing murmur heard best at the apex. His right elbow is tender on extension and flexion with mild swelling. Laboratory tests reveal a positive C-reactive protein, an ESR of 40 and a WBC of 22,000 with a left shift. ECG shows a prolonged PR interval. Which of the following tests would be positive for the microorganism responsible for this patient’s illness?
💡 Explanation
**Core Concept**
The patient's presentation suggests an acute rheumatic fever (ARF) caused by a post-streptococcal reaction. ARF is a complication of untreated or inadequately treated streptococcal pharyngitis, leading to an autoimmune response against the heart, joints, and other tissues.
**Why the Correct Answer is Right**
The patient's history of sore throat and rash, followed by symptoms of ARF, points towards a streptococcal infection. The positive throat culture for gram-positive cocci in chains further supports this. The correct test would be one that detects the presence of antibodies against the Group A beta-hemolytic streptococcus (GABHS) or the presence of the ASO (Anti-Streptolysin O) titer.
**Why Each Wrong Option is Incorrect**
* **Option A:** While blood cultures may reveal the causative organism, they are not the primary test for detecting the microorganism responsible for ARF.
* **Option B:** PCR (Polymerase Chain Reaction) may be used to detect the bacterial DNA, but it's not the most commonly used test for diagnosing ARF.
* **Option D:** The ECG findings of a prolonged PR interval are consistent with ARF, but it's not a test for the causative microorganism.
**Clinical Pearl / High-Yield Fact**
ARF is a preventable disease by administering antibiotics to patients with streptococcal pharyngitis within the first 9 days of illness. The Jones criteria are used to diagnose ARF, which include major and minor criteria.