**Core Concept**
Isotretinoin, a derivative of vitamin A, is a powerful oral retinoid used primarily for the treatment of severe acne. Its effectiveness is attributed to its ability to reduce sebum production, inhibit inflammation, and promote cell turnover.

**Why the Correct Answer is Right**
The maximum dose of isotretinoin should not exceed 2 mg/kg/day to minimize the risk of adverse effects, such as teratogenicity, hyperlipidemia, and pseudotumor cerebri. This dose is based on the drug's pharmacokinetics and the need to balance efficacy with safety. The optimal duration of treatment is typically 4-6 months, with a cumulative dose of 120-150 mg/kg being a common goal.

**Why Each Wrong Option is Incorrect**
**Option A:** There is no established upper limit for isotretinoin dose in terms of milligrams per day (mg/day) without considering the patient's weight.
**Option B:** A dose of 1 mg/kg/day may be insufficient for achieving therapeutic efficacy in many patients.
**Option C:** There is no evidence to support a maximum dose of 5 mg/kg/day, as this would increase the risk of adverse effects.

**Clinical Pearl / High-Yield Fact**
To avoid teratogenicity, female patients prescribed isotretinoin must use two forms of effective contraception simultaneously and undergo regular pregnancy testing.

**Correct Answer:** C. 2 mg/kg/day