**Core Concept**
Erythema nodosum is a type of skin inflammation characterized by the formation of tender, red nodules on the skin, typically on the shins. This condition is often associated with various underlying causes, including infections, medications, and autoimmune disorders.

**Why the Correct Answer is Right**
The correct answer, **D. Sulphonamides**, is a medication that can cause erythema nodosum as a rare side effect. This reaction is thought to be an allergic or idiosyncratic response to the medication, which can lead to the formation of the characteristic skin nodules. Sulphonamides are a class of antimicrobial agents that can cause a range of hypersensitivity reactions, including erythema nodosum.

**Why Each Wrong Option is Incorrect**
* **Option A:** Contraceptive pills are not typically associated with erythema nodosum, although they may cause other skin reactions such as chloasma or melasma.
* **Option B:** Barbiturates are a class of sedative medications that are not commonly linked to erythema nodosum.
* **Option C:** Penicillin is an antibiotic that can cause a range of hypersensitivity reactions, but erythema nodosum is not a typical side effect.

**Clinical Pearl / High-Yield Fact**
When considering the diagnosis of erythema nodosum, it is essential to take a thorough medical history to identify potential underlying causes, including medications, infections, and autoimmune disorders.

**Correct Answer: D. Sulphonamides**