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 ααααα – 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 ααααα – 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 showing α-haemolytic colonies of gram-positive cocci. This presentation suggests a severe infection likely caused by *Streptococcus pneumoniae*, a common pathogen in such clinical contexts.
## **Why the Correct Answer is Right**
*Streptococcus pneumoniae* is known for causing invasive disease, including bacteremia and pneumonia, particularly in infants and young children. The organism is characterized by its α-haemolytic activity on blood agar, producing greenish discoloration around colonies. The key feature of *S. pneumoniae* relevant to this question is its susceptibility to optochin (ethylhydrocupreine hydrochloride), which is used as a screening test for identification. *S. pneumoniae* is sensitive to optochin, which inhibits its growth.
## **Why Each Wrong Option is Incorrect**
- **Option A:** This option would be incorrect if it represented an agent to which *S. pneumoniae* is not typically susceptible, such as an aminoglycoside, which might not be the first choice for treating pneumococcal infections due to its limited activity against gram-positive cocci in general and potential for resistance.
- **Option B:** If this option represented vancomycin, while *S. pneumoniae* is susceptible to vancomycin, the use of vancomycin sensitivity as a screening test is not specific or practical for initial identification compared to optochin sensitivity.
- **Option D:** This option might represent a different antibiotic; if *S. pneumoniae* is not characteristically susceptible or if the agent is not used for identification purposes, it would be incorrect.
## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that optochin sensitivity is a rapid and useful tool for identifying *Streptococcus pneumoniae* in the laboratory. Adding a drop of optochin to a suspected *S. pneumoniae* colony on an agar plate and observing a zone of inhibition around the colony is a practical method for confirming the identification.
## **Correct Answer:** B.
✓ Correct Answer: C. Optochin
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