**Core Concept**
Isotretinoin is a synthetic retinoid, derived from vitamin A, used primarily in the treatment of severe acne, particularly acne conglobata and nodulosis. It is known for its teratogenic effects and strict prescribing guidelines.

**Why the Correct Answer is Right**
Isotretinoin works by inhibiting sebaceous gland function, reducing sebum production, and subsequently decreasing the number of comedones (blackheads and whiteheads) and inflammatory lesions. It achieves this by binding to retinoic acid receptors (RARs) and retinoid X receptors (RXRs), which modulate gene expression involved in skin cell differentiation and proliferation.

**Why Each Wrong Option is Incorrect**
**Option A:** Isotretinoin is not a type of steroid, which is a different class of medications used for various dermatological and other conditions.
**Option B:** Isotretinoin is not primarily used for treating psoriasis, although it may be used in some cases; it is mainly used for severe acne.
**Option C:** Isotretinoin is not a form of vitamin B, but rather a synthetic derivative of vitamin A.

**Clinical Pearl / High-Yield Fact**
Isotretinoin is known for its teratogenic effects, and female patients must use two forms of contraception while taking isotretinoin and for one month after stopping the medication to prevent pregnancy.

**Correct Answer:** B. A medication used primarily in the treatment of severe acne, particularly acne conglobata and nodulosis.