**Question:** Drug used in actinic keratosis is:

A. 5-Fluorouracil (5-FU)
B. Imiquimod
C. Soriatane (ACitR - acitretin)
D. Fluorouracil (5-FU)

**Core Concept:**
Actinic keratosis (AK) is a precursor lesion of squamous cell carcinoma (SCC) caused by chronic sun exposure or immunosuppression. AKs are typically rough, scaly, and red patches on sun-exposed skin. They may progress to invasive SCC if left untreated. Treatment options include topical, intralesional, or systemic therapies.

**Why the Correct Answer is Right:**
Actinic keratosis is treated with topical 5-fluorouracil (5-FU) because it inhibits the enzyme thymidylate synthase, leading to inhibition of DNA synthesis and cell proliferation. Topical 5-FU is effective in treating AKs due to its selective cytotoxicity towards rapidly dividing cells.

**Why Each Wrong Option is Incorrect:**

A. Imiquimod: This is a topical immune response modifier (IMDM) which stimulates the immune system to recognize and eliminate AK cells, but it is not as potent as 5-FU for AK treatment.
B. Fluorouracil (5-FU) is incorrect as it is a nucleoside analogue that interferes with DNA synthesis, whereas 5-FU is a pyrimidine analogue that inhibits thymidylate synthase.
C. Soriatane (ACitR - acitretin): This is a retinoid used for the treatment of psoriasis and is not specifically designed for actinic keratosis.

**Clinical Pearl:**

5-Fluorouracil (5-FU) is the correct answer because of its selective cytotoxicity towards rapidly dividing cells, making it highly effective for the treatment of actinic keratosis. Imiquimod is a reasonable option but may not be as potent as 5-FU. Soriatane (ACitR) is incorrect, as it is used for psoriasis treatment and not specifically for actinic keratosis. Fluorouracil is incorrect due to its different chemical structure and mechanism of action compared to 5-FU.