An elderly diabetic has left sided orbital cellulitis, CT scan of paranasal sinuses shows evidence of left maxillary sinusitis. A Gram-stained smear of the orbital exudates shows irregularly branching septate hyphae. The following is the most likely aetiological agent.
**Core Concept**
Orbital cellulitis is a serious infection involving the orbital tissues, often extending from adjacent structures such as the paranasal sinuses. In diabetic patients, fungal infections are a significant concern, particularly those with irregularly branching hyphae.
**Why the Correct Answer is Right**
The presence of irregularly branching septate hyphae in the Gram-stained smear is indicative of a fungal infection. In this case, the most likely aetiological agent is *Aspergillus fumigatus*. This fungus is commonly found in soil and decaying organic matter, and it can cause invasive aspergillosis in immunocompromised individuals, including those with diabetes. **Aspergillus** hyphae are characterized by their irregular branching pattern, which is distinct from the regular, parallel branches seen in *Mucorales*.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because *Mucorales*, such as *Mucor* and *Rhizopus*, typically present with non-septate (pauciseptate) hyphae, which is not consistent with the description of irregularly branching septate hyphae.
**Option B:** This option is incorrect because *Candida* species typically present with pseudohyphae and blastoconidia, which is not consistent with the description of irregularly branching septate hyphae.
**Option C:** This option is incorrect because *Blastomyces dermatitidis* typically presents with broad-based budding yeast forms, which is not consistent with the description of irregularly branching septate hyphae.
**Option D:** This option is incorrect because *Histoplasma capsulatum* typically presents with small, intracellular yeast forms, which is not consistent with the description of irregularly branching septate hyphae.
**Clinical Pearl / High-Yield Fact**
In diabetic patients, fungal orbital cellulitis is a medical emergency, and prompt antifungal therapy is essential to prevent vision loss and other complications.
**Correct Answer:** C. *Histoplasma capsulatum*