**Core Concept**
The patient's presentation of rash, mouth pain, and specific oral lesions suggests a systemic condition with mucocutaneous manifestations. The violaceous, sharply demarcated papules with fine white lines in a lacy pattern are characteristic of a specific dermatological condition.

**Why the Correct Answer is Right**
The patient's symptoms, particularly the oral lesions with fine white reticulation, are consistent with erythema multiforme (EM). EM is a hypersensitivity reaction to various triggers, including infections (such as hepatitis C), medications, and other systemic conditions. The lesions in EM are typically targetoid or iris-like and can be accompanied by oral mucosal involvement. The patient's hepatitis C infection is a likely trigger for the EM.

**Why Each Wrong Option is Incorrect**
**Option A:** Stevens-Johnson syndrome (SJS) is a more severe form of EM, characterized by extensive skin and mucosal involvement, and is often associated with medication reactions. The patient's presentation does not meet the criteria for SJS.
**Option B:** Pemphigus vulgaris is an autoimmune blistering disorder that typically presents with oral mucosal erosions and skin lesions, but the characteristic "chicken skin" appearance and Nikolsky's sign are absent in this case.
**Option C:** Lichen planus is a chronic inflammatory condition that can affect the skin and mucous membranes, but it typically presents with violaceous, polygonal papules and not the characteristic lacy pattern seen in this patient.

**Clinical Pearl / High-Yield Fact**
Erythema multiforme can be triggered by various infections, including hepatitis C, and is often associated with a history of recent viral infections. The characteristic oral lesions with fine white reticulation are a key diagnostic clue.

**Correct Answer: C. Erythema multiforme**