**Core Concept**
Drug-induced lupus (DIL) is a rare, autoimmune disorder triggered by certain medications, characterized by the presence of autoantibodies against nuclear antigens, particularly antihistone antibodies. The clinical presentation of DIL is similar to systemic lupus erythematosus (SLE), but it typically resolves after discontinuation of the offending drug.

**Why the Correct Answer is Right**
The antihistone antibody is a specific marker for drug-induced lupus, as it is present in the vast majority of patients with DIL. This is in contrast to SLE, where antihistone antibodies are present in only a minority of patients. The presence of antihistone antibodies in a patient with a history of recent medication exposure is highly suggestive of DIL.

**Why Each Wrong Option is Incorrect**
**Option A:** Anti-SSA/Ro and anti-SSB/La antibodies are more commonly associated with Sjögren's syndrome and SLE, respectively.
**Option B:** Anti-Sm antibodies are highly specific for SLE, but not for DIL.
**Option C:** Rheumatoid factor is not a specific marker for DIL and can be present in a variety of autoimmune disorders.

**Clinical Pearl / High-Yield Fact**
Remember that DIL typically resolves after discontinuation of the offending drug, and the presence of antihistone antibodies is a key distinguishing feature from SLE.

**Correct Answer:** A. Antihistone antibody.