## **Core Concept**
The question describes an infant with high-grade fever and respiratory distress, with a blood culture showing alpha-hemolytic, gram-positive cocci. This presentation suggests a severe infection, likely pneumonia, caused by *Streptococcus pneumoniae*, a common pathogen in such clinical scenarios.

## **Why the Correct Answer is Right**
*Streptococcus pneumoniae* is known for being alpha-hemolytic and gram-positive. In the screening test for identification, one key susceptibility test is used to differentiate it from other streptococci: its sensitivity to optochin (ethylhydrocupreine). Optochin sensitivity is a specific test for *S. pneumoniae*, as most strains are sensitive to optochin. This is a rapid and cost-effective method for presumptive identification.

## **Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because while *S. pneumoniae* can cause severe infections, the specific susceptibility pattern mentioned here does not directly relate to a commonly used screening test for this organism.
* **Option B:** This option refers to a susceptibility that might not be specific or commonly used for the identification of *S. pneumoniae* in clinical practice, especially not as directly related as optochin sensitivity.
* **Option C:** This option might relate to other bacteria or susceptibility patterns not specifically used for identifying *S. pneumoniae*.
* **Option D:** This option does not directly relate to a commonly used screening test for *S. pneumoniae*.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that *Streptococcus pneumoniae* is typically sensitive to optochin. This is a high-yield fact for medical students and residents, especially in the context of identifying pathogens from blood cultures or respiratory specimens. Remembering that optochin sensitivity can quickly help identify *S. pneumoniae* can guide clinical decision-making.

## **Correct Answer:** .