**Core Concept**
Glucocorticoids like hydrocortisone are steroid anti-inflammatory agents that suppress the immune system and reduce inflammation. They are commonly used in dermatology to treat various skin conditions, including allergic reactions and exanthematous skin eruptions.

**Why the Correct Answer is Right**
Hydrocortisone works by inhibiting the release of inflammatory mediators from mast cells and other immune cells. This action prevents the typical skin manifestations associated with allergic reactions, such as redness, swelling, and exanthematous eruptions. In contrast to other medications listed, hydrocortisone is not known to induce skin eruptions as a side effect.

**Why Each Wrong Option is Incorrect**
**Option A:** Phenytoin is an antiepileptic medication that can cause a range of skin reactions, including Stevens-Johnson syndrome, a severe form of exanthematous skin eruption. Phenytoin's metabolites can also induce skin eruptions in susceptible individuals.
**Option C:** Ampicillin is a penicillin antibiotic that can cause anaphylaxis or other allergic reactions in some patients, including exanthematous skin eruptions.
**Option D:** Phenylbutazone is a nonsteroidal anti-inflammatory drug (NSAID) that can cause various types of skin reactions, including exanthematous eruptions, as part of its side effect profile.

**Clinical Pearl / High-Yield Fact**
Glucocorticoids like hydrocortisone can be used to treat allergic reactions, including anaphylaxis, and are often the first line of treatment for acute skin reactions. However, long-term use of these medications can lead to steroid dependency and other complications.

**✓ Correct Answer: B. Hydrocoisone**