**Core Concept**
The question is testing the ability to diagnose a skin infection based on laboratory findings. Group A streptococcal pyoderma is a type of skin infection caused by the bacterium *Streptococcus pyogenes*, which is a Gram-positive coccus. The presence of hemolytic colonies on blood agar is a key characteristic of this organism.

**Why the Correct Answer is Right**
The correct answer is based on the identification of the bacterium as a Gram-positive coccus with hemolytic colonies on blood agar. This is a classic laboratory finding for *Streptococcus pyogenes*, which is the causative agent of Group A streptococcal pyoderma. The hemolysis is due to the production of hemolysin, which breaks down red blood cells in the agar.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify the type of streptococcal infection. While Group A streptococcal pyoderma is a common cause of skin infections in children, other types of streptococci can also cause similar infections.
* **Option B:** This option is incorrect because it does not mention the laboratory findings that are characteristic of Group A streptococcal pyoderma. While clinical presentation can be suggestive, laboratory confirmation is essential for a definitive diagnosis.
* **Option C:** This option is incorrect because it is too broad and does not specify the type of streptococcal infection. Many types of streptococci can cause different types of infections, and this option does not provide enough information to make a specific diagnosis.

**Clinical Pearl / High-Yield Fact**
It's essential to note that Group A streptococcal pyoderma can be confused with other types of skin infections, such as impetigo or folliculitis. However, the presence of hemolytic colonies on blood agar is a key characteristic that can help differentiate it from other infections.

**Correct Answer: D. Lancefield grouping of the organism with antisera specific to Group A streptococci.**