**Question:** An outbreak of streptococcal pharyngitis has occurred in a remote village. In order to carry out the epidemiological investigations of the outbreak, it is necessary to perform the culture of the throat swab of the patients suffering from the disease. The transposal media of choice would be -

A. Blood agar
B. Chocolate agar
C. MacConkey agar
D. Throat swab culture without selecting a transposal media

**Correct Answer:** C. MacConkey agar

**Core Concept:**
In the context of investigating an outbreak of streptococcal pharyngitis, throat swab culture is a crucial step to isolate the causative agent, Streptococcus pyogenes (group A beta-hemolytic Streptococcus). This bacterium is known to be responsible for causing streptococcal pharyngitis, also known as scarlet fever. The primary objective of performing the throat swab culture is to identify the bacteria and determine its susceptibility to various antibiotics, which is essential for appropriate treatment.

**Why the Correct Answer is Right:** MacConkey agar is a selective and differential medium used for the identification of Gram-negative bacteria. In this context, MacConkey agar is the preferred choice among the options provided because it helps to differentiate between Gram-positive and Gram-negative bacteria. Streptococcus pyogenes, the causative agent of streptococcal pharyngitis, is a Gram-positive coccus. Therefore, MacConkey agar helps to prevent contamination from Gram-negative bacteria, which are less likely to be present in throat swab cultures from patients with streptococcal pharyngitis.

**Why Each Wrong Option is Incorrect:**

A. Blood agar is an agar base used for the isolation and cultivation of bacteria, including Gram-positive cocci. However, it does not provide selective properties for the isolation of Streptococcus pyogenes specifically.

B. Chocolate agar is another agar base used for the isolation and cultivation of bacteria. It is not specifically designed to isolate Streptococcus pyogenes.

D. Performing a throat swab culture without selecting a transposal medium will result in a higher likelihood of contamination from other bacteria and possibly viruses, making the results less reliable and less informative for identifying the causative agent and determining antibiotic susceptibility.

**Clinical Pearl:**

In practice, it is essential to use selective media like MacConkey agar when investigating an outbreak of streptococcal pharyngitis, ensuring a higher likelihood of isolating Streptococcus pyogenes and reducing contamination from other bacterial species or viruses. This will provide accurate information regarding antibiotic susceptibility, allowing for appropriate treatment of the affected individuals and preventing potential spread within the community.