**Core Concept**
Rheumatoid arthritis (RA) is a chronic autoimmune disorder characterized by inflammation and joint destruction. Systemic steroids are potent anti-inflammatory agents used to manage severe forms of RA. In this context, lung involvement and mononeuritis multiplex are both serious complications that warrant the use of systemic steroids.

**Why the Correct Answer is Right**
Lung involvement in RA, such as interstitial lung disease, can lead to significant morbidity and mortality. Systemic steroids are the mainstay of treatment for pulmonary involvement, as they can rapidly reduce inflammation and prevent further lung damage. Mononeuritis multiplex, a condition characterized by the simultaneous involvement of multiple peripheral nerves, is another indication for systemic steroids in RA. This condition can lead to significant morbidity, including pain, weakness, and paralysis, and requires prompt treatment with systemic steroids to prevent long-term damage.

**Why Each Wrong Option is Incorrect**
**Option A:** Carpal tunnel syndrome is a common complication of RA, but it is typically managed with local treatments such as wrist splints, corticosteroid injections, or surgery. Systemic steroids are not usually required for carpal tunnel syndrome.

**Option B:** While carpel tunnel syndrome is a common complication of RA, it is not a definite indication for systemic steroids. Local treatments are usually sufficient to manage this condition.

**Clinical Pearl / High-Yield Fact**
In patients with RA, the presence of lung involvement or mononeuritis multiplex is a medical emergency that requires prompt treatment with systemic steroids. These conditions can rapidly progress and lead to significant morbidity and mortality if left untreated.

**Correct Answer:**
✓ Correct Answer: D. Both 'a' & 'c'