**Core Concept**
The patient's presentation is suggestive of a scalp infection, likely folliculitis or furuncles, which may have progressed to a more severe condition such as carbuncles or hidradenitis suppurativa. The presence of boggy swelling, multiple sinuses, and purulent discharge points towards a deep-seated infection involving the hair follicles.

**Why the Correct Answer is Right**
The most helpful diagnostic evaluation would be a **Complete Blood Count (CBC)**, including a differential count, to assess for signs of infection or inflammation. This would help identify any potential underlying conditions such as neutropenia, which could predispose the patient to severe infections. Additionally, a CBC can provide information on the patient's overall health status and help guide further management.

**Why Each Wrong Option is Incorrect**
**Option A:** A **Skin biopsy** may be helpful in cases where the diagnosis is unclear or if there is suspicion of a more complex condition such as hidradenitis suppurativa, but it is not the most helpful initial diagnostic test.

**Option B:** A **Chest X-ray** may be indicated if there is suspicion of a systemic infection or if the patient has a history of respiratory symptoms, but it is not directly relevant to the scalp infection.

**Option C:** A **Liver function test (LFT)** may be indicated if there is suspicion of a systemic infection or liver involvement, but it is not the most helpful initial diagnostic test for a scalp infection.

**Clinical Pearl / High-Yield Fact**
A boggy swelling of the scalp is often associated with a deeper infection, such as a carbuncle, which can spread to the lymph nodes and cause significant morbidity.

**Correct Answer:** C.