## **Core Concept**
Blastomyces dermatitidis is a pathogenic fungus that causes blastomycosis, primarily affecting the lungs. The infection typically occurs through inhalation of spores. This fungus is endemic in certain regions, particularly in North America.

## **Why the Correct Answer is Right**
The correct answer, **lungs**, is the most common portal of entry for Blastomyces dermatitidis because the infection usually begins when spores of the fungus are inhaled into the lungs. This inhalation leads to the initial pulmonary infection, which can then disseminate to other parts of the body if left untreated. The lungs serve as the primary site for the initial immune response to the fungus.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The skin is not the primary portal of entry for Blastomyces dermatitidis. While skin lesions can occur as a result of disseminated infection, the skin is not where the infection typically starts.
- **Option B:** The gastrointestinal tract is not a recognized portal of entry for Blastomyces dermatitidis. Infections through this route are not documented for this particular fungus.
- **Option C:** The eyes are not a common portal of entry for Blastomyces dermatitidis. Ocular involvement can occur in the context of disseminated disease but is not the initial site of infection.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that blastomycosis often presents with pulmonary symptoms, such as cough and fever, but can also have skin, bone, and genitourinary manifestations. Given its endemic areas, a travel history or residence in regions like the Mississippi River Valley, parts of Canada, and other areas near waterways can increase the suspicion for this diagnosis.

## **Correct Answer:** . lungs