A 25 years old female complains of recurrent rhinitis, nasal discharge and bilateral nasal blockage since one year. She has the history of asthma and allergy. On examination, multiple ethmoidal polyps are noted with mucosal thickening and impacted secretions in both the nasal cavities. A biopsy is taken and the material is cultured which shown the growth of many non pigmented septate hyphae with dichotomous branching typically at 45degrees. Which of the following is the most likely responsible organism

**Core Concept**
Aspergillus fumigatus is a common opportunistic pathogen that causes allergic reactions and infections in immunocompromised individuals. It is known to produce conidia that can cause allergic sinusitis and invasive aspergillosis. The organism is characterized by its non-pigmented, septate hyphae with dichotomous branching at 45 degrees.

**Why the Correct Answer is Right**
The presence of non-pigmented, septate hyphae with dichotomous branching at 45 degrees is a classic microscopic feature of Aspergillus species. Aspergillus fumigatus is the most common species responsible for causing allergic sinusitis and is often associated with asthma and allergic reactions. The organism can produce enzymes such as proteases and elastases that contribute to tissue damage and inflammation. The growth of Aspergillus fumigatus in the nasal cavity can lead to the formation of ethmoidal polyps, mucosal thickening, and impacted secretions, as seen in this patient.

**Why Each Wrong Option is Incorrect**

**Option B:** Rhizopus and Mucor are both part of the Mucorales order of fungi, which are characterized by their non-septate (coenocytic) hyphae, not septate hyphae as seen in Aspergillus. They are also typically pigmented and have a different branching pattern.

**Option C:** Mucor is a genus within the Mucorales order, which is also characterized by non-septate hyphae and a different branching pattern. It is more commonly associated with invasive mucormycosis, particularly in immunocompromised patients.

**Option D:** Candida is a genus of yeast that is typically characterized by its budding yeast form and pseudohyphae, not septate hyphae with dichotomous branching.

**Clinical Pearl / High-Yield Fact**
Aspergillus fumigatus is a common cause of allergic sinusitis and can be associated with asthma and allergic reactions. The organism can produce enzymes that contribute to tissue damage and inflammation, leading to the formation of ethmoidal polyps and mucosal thickening.

**✓ Correct Answer:** A. **Aspergillus fumigatus**

✓ Correct Answer: A. Aspergillus fumigatus