An infant had high grade fever and respiratory distress at the time of presentation to the emergency room. The sample collected for blood culture was subsequently positive showing growth of a haemolytic colonies. On Gram staining these were gram positive cocci. In the screening test for identification, the suspected pathogen is likely to be susceptible to the following agent:
An infant had high grade fever and respiratory distress at the time of presentation to the emergency room. The sample collected for blood culture was subsequently positive showing growth of a haemolytic colonies. On Gram staining these were gram positive cocci. In the screening test for identification, the suspected pathogen is likely to be susceptible to the following agent:
💡 Explanation
## **Core Concept**
The question describes a clinical scenario involving an infant with high-grade fever and respiratory distress, with a blood culture positive for gram-positive cocci showing hemolytic colonies. This presentation suggests a severe infection, likely pneumonia or sepsis, caused by a virulent pathogen. The key here is the identification of gram-positive cocci that produce hemolytic colonies, which narrows down the differential diagnoses.
## **Why the Correct Answer is Right**
The description points towards *Streptococcus pneumoniae*, a common cause of severe infections like pneumonia, especially in infants and young children. *S. pneumoniae* is known for producing alpha-hemolytic colonies on blood agar, which are greenish in appearance due to the partial breakdown of hemoglobin. A key screening test for the identification of *S. pneumoniae* involves its susceptibility to optochin (ethylhydrocupreine hydrochloride). *S. pneumoniae* is characteristically sensitive to optochin.
## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because the agent mentioned here does not directly correspond with common screening tests for the described pathogen.
- **Option B:** This option refers to an agent that might not specifically relate to the common identification methods for gram-positive cocci causing hemolytic colonies in this context.
- **Option C:** Bacitracin sensitivity is used to differentiate *Streptococcus pyogenes* (Group A beta-hemolytic streptococci) from other beta-hemolytic streptococci. *S. pyogenes* is sensitive to bacitracin, but it does not match the description of alpha-hemolytic colonies.
- **Option D:** This option does not directly relate to the specific identification of *S. pneumoniae* or other alpha-hemolytic streptococci.
## **Clinical Pearl / High-Yield Fact**
A critical clinical pearl is that *Streptococcus pneumoniae* can cause severe infections, including pneumonia, meningitis, and bacteremia, especially in vulnerable populations like infants and the elderly. The organism's identification can often be quickly narrowed down by its alpha-hemolytic pattern on blood agar and its susceptibility to optochin.
## **Correct Answer:** .
✓ Correct Answer: C. Optochin
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