**Core Concept**
Inhaled corticosteroids like beclomethasone dipropionate suppress inflammation in the airways but can also have local side effects due to their high concentration at the site of administration. These side effects are often related to the mucosal effects of corticosteroids.

**Why the Correct Answer is Right**
Oropharyngeal candidiasis, also known as thrush, is a common side effect of inhaled corticosteroids like beclomethasone dipropionate. This occurs because the inhaled corticosteroid suppresses the local immune response, allowing fungal overgrowth. The dipropionate ester is more lipophilic and has a higher affinity for the glucocorticoid receptor, increasing its local effects.

**Why Each Wrong Option is Incorrect**
* **Option A:** Pneumonia is not a common side effect of inhaled beclomethasone dipropionate. While inhaled corticosteroids can increase the risk of pneumonia in some cases, this is not the most common side effect.
* **Option C:** Atrophic rhinitis is not a typical side effect of inhaled beclomethasone dipropionate. Atrophic rhinitis is more commonly associated with nasal decongestants or topical nasal anesthetics.
* **Option D:** Pituitary adrenal suppression is a systemic side effect of corticosteroids but is less common with inhaled beclomethasone dipropionate due to its local action and lower systemic bioavailability.

**Clinical Pearl / High-Yield Fact**
When prescribing inhaled corticosteroids like beclomethasone dipropionate, it's essential to counsel patients on the risk of oropharyngeal candidiasis and to monitor for symptoms such as white patches in the mouth or throat.

**✓ Correct Answer: B. Oropharyngeal candidiasis**