**Core Concept**
The patient is presenting with symptoms of acute sinusitis, which is a common pediatric condition characterized by inflammation of the paranasal sinuses. This condition often results from a viral upper respiratory tract infection, but can also be caused by bacterial pathogens.

**Why the Correct Answer is Right**
The patient's symptoms, including purulent nasal discharge, fever, conjunctival congestion, and opacification of the ethmoid sinus on X-ray, are consistent with acute sinusitis. Given the negative culture result from the eye discharge, it is essential to further evaluate the patient's nasal discharge for bacterial pathogens. A crucial next step in evaluating this patient is to obtain a sample of nasal discharge for culture and sensitivity testing to guide antibiotic therapy.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scan of the sinuses may be considered in patients with suspected chronic sinusitis or complications of sinusitis, but it is not the next step in evaluating this patient with acute symptoms.

**Option B:** Blood cultures may be indicated in patients with suspected bacterial meningitis or sepsis, but they are not the next step in evaluating this patient with localized symptoms of sinusitis.

**Option C:** A throat swab may be indicated in patients with suspected streptococcal pharyngitis, but it is not the next step in evaluating this patient with symptoms of sinusitis.

**Option D:** A complete blood count (CBC) with differential may provide information on the patient's overall inflammatory response, but it is not the next step in evaluating this patient with localized symptoms of sinusitis.

**Clinical Pearl / High-Yield Fact**
In pediatric patients with suspected acute sinusitis, it is essential to consider the possibility of complications such as orbital cellulitis or intracranial abscess, particularly if the patient's symptoms worsen or if there is a delay in obtaining cultures.

**Correct Answer: C. A throat swab may be indicated in patients with suspected streptococcal pharyngitis, but it is not the next step in evaluating this patient with symptoms of sinusitis.**