**Core Concept**
The clinical presentation suggests a diagnosis of folliculitis or possibly a more severe condition like follicular or scalp abscess. The presence of painful boggy swelling, sinuses with purulent discharge, and easily pluckable hairs points towards an infectious process involving the hair follicles.

**Why the Correct Answer is Right**
The most helpful diagnostic evaluation for this condition would be a **Gram stain**. This is because a Gram stain can quickly identify the causative organism of the infection, which is likely to be Staphylococcus aureus, a gram-positive bacterium. The Gram stain can differentiate between gram-positive and gram-negative bacteria, which is crucial in guiding the treatment. In this case, a gram-positive organism would suggest a more likely diagnosis of folliculitis or a scalp abscess, and the treatment would focus on antibiotics effective against gram-positive bacteria.

**Why Each Wrong Option is Incorrect**
**Option A:** A complete blood count (CBC) would not provide specific information about the causative organism or the diagnosis. It might show signs of infection like elevated white blood cell count, but it would not be helpful in guiding the treatment.
**Option B:** A biopsy might be helpful in excluding other conditions like fungal infections or autoimmune disorders, but it would not be the most helpful initial diagnostic test for a suspected folliculitis or scalp abscess.
**Option C:** An MRI or CT scan might be helpful in evaluating the extent of the infection or in cases where there is suspicion of a more complex condition like a subperiosteal abscess, but it would not be the most helpful initial diagnostic test.

**Clinical Pearl / High-Yield Fact**
In cases of suspected folliculitis or scalp abscess, it is essential to remember that the treatment should focus on antibiotics effective against gram-positive bacteria, and the patient should be evaluated for any signs of systemic infection.

**Correct Answer:** C.