**Core Concept**
Clostridium perfringens is a gram-positive, anaerobic, spore-forming bacterium that can cause food poisoning, gas gangrene, and other infections. In this case, the patient's symptoms and lab results suggest an infection caused by Clostridium perfringens.

**Why the Correct Answer is Right**
Clostridium perfringens produces several toxins, including alpha-toxin, which causes tissue damage and necrosis. The presence of this bacterium in the patient's blood indicates bacteremia, a serious condition that requires prompt treatment. IV penicillin is the recommended treatment for Clostridium perfringens infections, as it is effective against the bacterium. The patient should return for IV penicillin therapy to receive appropriate treatment and prevent complications.

**Why Each Wrong Option is Incorrect**
**Option A:** IV penicillin therapy plus colonoscopy is not indicated in this case, as the patient's symptoms are not suggestive of a gastrointestinal tract issue that would require a colonoscopy. Additionally, colonoscopy is not a treatment for Clostridium perfringens infections.

**Option B:** Returning for blood culture is not the next step, as the patient's blood culture has already been sent and the results are available. The focus should now be on treating the infection rather than repeating diagnostic tests.

**Option D:** IV penicillin therapy plus echocardiography is not indicated, as there is no evidence to suggest cardiac involvement or a need for echocardiography in this case.

**Clinical Pearl / High-Yield Fact**
Clostridium perfringens infections can be prevented by proper food handling and storage, as well as vaccination against tetanus, which can reduce the risk of tetanus toxin-mediated infections caused by this bacterium.

**✓ Correct Answer: C. Return for IV penicillin therapy.**