**Core Concept**
A vesicle over the lip in a 12-year-old boy suggests a diagnosis of herpes labialis, a common viral infection caused by the herpes simplex virus (HSV). This condition is characterized by the formation of vesicles or blisters on the lips, which rupture to form ulcers.

**Why the Correct Answer is Right**
The correct investigation for herpes labialis is a viral culture or PCR (polymerase chain reaction) to confirm the presence of HSV. This can be done by taking a sample from the affected area, which is then sent to a laboratory for analysis. The PCR test is highly sensitive and can detect even small amounts of viral DNA. In contrast, a viral culture may take longer to produce results.

**Why Each Wrong Option is Incorrect**
* **Option A:** Blood tests are not typically used to diagnose herpes labialis, as they are not very specific for this condition.
* **Option B:** A skin biopsy may be used to diagnose other skin conditions, but it is not the best initial test for herpes labialis.
* **Option C:** A complete blood count (CBC) may be used to evaluate for signs of infection or inflammation, but it is not specific for diagnosing herpes labialis.

**Clinical Pearl / High-Yield Fact**
It's essential to note that herpes labialis is contagious, and individuals with active lesions should avoid close contact with others to prevent transmission.

**Correct Answer:** C.