Okay, let's tackle this question. The scenario is a 12-year-old boy with a vesicle on his lip. The question is asking for the appropriate investigation.

First, I need to recall the common causes of vesicles on the lip in a child. Vesicles can be caused by various conditions, but in a child, herpes simplex virus (HSV) is a common culprit. Herpes labialis, or cold sores, typically presents with clusters of small vesicles, often preceded by a tingling or burning sensation. Another possibility might be impetigo, but that's more associated with crusted lesions. Contact dermatitis could also cause blistering, but that's usually due to an allergic reaction and might present with more redness and itching.

The correct investigation for HSV would be a Tzanck smear or viral culture. However, a Tzanck smear is a rapid test that looks for multinucleated giant cells, which are characteristic of HSV and varicella-zoster virus (VZV). Alternatively, PCR testing for HSV DNA is more specific. Another option might be a viral culture, but that's less commonly used now due to the availability of PCR. If the vesicle is suspected of being from another cause, like a bacterial infection, a Gram stain or culture might be considered, but the presentation here is more typical of HSV.

Looking at the options provided, even though the specific investigations aren't listed, the correct answer would likely be a Tzanck smear or HSV PCR. The other options, like CBC, skin biopsy, or allergy testing, don't fit as well. A CBC might show elevated WBCs in bacterial infections but isn't specific. A skin biopsy is more invasive and not typically needed for a straightforward HSV case. Allergy testing would be for contact dermatitis, which is less likely here.

So the core concept is identifying the cause of vesicles on the lip in a child, with HSV being the most probable. The correct investigation would be one that detects HSV, such as Tzanck smear or PCR. The other options are either not specific or not relevant to the most likely diagnosis.

**Core Concept**
Vesicles on the lip in a child are most commonly caused by herpes simplex virus (HSV) infection, particularly HSV-1. Diagnosis relies on identifying characteristic multinucleated giant cells via Tzanck smear or confirming HSV DNA via PCR.

**Why the Correct Answer is Right**
HSV-1 typically presents with clusters of painful vesicles on the lip (herpes labialis). A Tzanck smear involves scraping cells from the base of the vesicle and staining for multinucleated giant cells, which are pathognomonic for HSV or VZV. PCR testing is more sensitive and specific but may require laboratory access. These investigations are non-invasive, rapid, and confirmatory for viral etiology.

**Why Each Wrong Option is Incorrect**
**Option A:** *CBC* is nonspecific and does not differentiate viral from bacterial causes.
**Option B:** *Skin biopsy* is unnecessary for typical HSV and reserved for atypical or persistent lesions.
**Option C:** *Allergy testing* is irrelevant unless there’s a history of contact dermatitis with crusted lesions.

**Clinical Pearl / High-Yield Fact**
HSV-1 is the most common cause of lip vesicles in children. Remember the "herpes triad

✓ Correct Answer: A. Tzank smear