**Core Concept:** Chronic asthma treated with steroids may lead to opportunistic infections due to suppression of the immune system. Organisms causing such infections include Mycobacterium tuberculosis (TB), which is gram-positive and weakly acid-fast.

**Why the Correct Answer is Right:**

The correct answer is **D. Mycobacterium tuberculosis** because:

1. Mycobacterium tuberculosis is a gram-positive bacillus and weakly acid-fast, matching the description provided in the question.
2. The patient has been taking steroids for years, which suppresses the immune system, making them more susceptible to opportunistic infections like tuberculosis.
3. The patient exhibits symptoms like productive cough, weight loss, night sweats, and imaging findings of abscesses in the lungs and brain, which are consistent with TB infection.

**Why Each Wrong Option is Incorrect:**

Option A (Staphylococcus aureus) is a common respiratory pathogen but is not weakly acid-fast.
Option B (Pseudomonas aeruginosa) is a gram-negative bacterium and does not match the description provided.
Option C (Candida albicans) is a fungal pathogen, not a bacterial one.

**Clinical Pearl:**

This case highlights the importance of considering opportunistic infections in immunocompromised patients, especially those on long-term steroid therapy. Early recognition and prompt initiation of appropriate treatment can significantly improve the patient's outcome.