**Core Concept**
The question is testing the student's knowledge of drug-induced skin reactions, a common adverse effect of medications. Drug-induced skin reactions can be categorized into various types, including allergic reactions, phototoxicity, and fixed drug eruptions.

**Why the Correct Answer is Right**
The most common drug-induced skin reaction is an allergic reaction, specifically a type IV hypersensitivity reaction. This reaction occurs when the immune system mistakenly identifies a drug molecule as a foreign substance, triggering the release of histamine and other chemical mediators. This leads to symptoms such as pruritus, erythema, and urticaria. The exact mechanism of type IV hypersensitivity involves the activation of T lymphocytes, which recognize the drug molecule through major histocompatibility complex (MHC) molecules.

**Why Each Wrong Option is Incorrect**
**Option A:** Phototoxicity is a type of skin reaction caused by the interaction of a drug with sunlight, leading to damage to skin cells. While it is an important adverse effect, it is not the most common drug-induced skin reaction.

**Option B:** Fixed drug eruptions are a type of skin reaction characterized by the recurrence of lesions at the same site after re-exposure to the offending drug. They are relatively rare compared to allergic reactions.

**Option C:** Stevens-Johnson syndrome (SJS) is a severe skin and mucous membrane disorder caused by a drug reaction. It is a rare but life-threatening condition, and not the most common drug-induced skin reaction.

**Clinical Pearl / High-Yield Fact**
To remember the most common drug-induced skin reaction, use the mnemonic "A.L.E.R.T." - Allergic reactions are the most common type of drug-induced skin reaction, followed by phototoxicity, fixed drug eruptions, and rare conditions like Stevens-Johnson syndrome.

**Correct Answer:** A. Allergic reactions.