**Core Concept**
A bunion is a deformity of the joint at the base of the big toe, characterized by a bony lump or protrusion on the medial aspect of the first metatarsal head. This condition is often associated with hallux valgus, a condition where the big toe points towards the second toe, causing the first metatarsal bone to deviate outward.

**Why the Correct Answer is Right**
The primary cause of bunions is the abnormal alignment of the first metatarsal bone and the big toe joint, which can be due to genetic predisposition, poorly fitting shoes, or other factors that put pressure on the toes. As a result, the medial aspect of the first metatarsal head undergoes osteoarthritic changes, leading to the formation of a bony lump.

**Why Each Wrong Option is Incorrect**
**Option A:** **Hallux varus** is a condition where the big toe points towards the midline of the body, not the second toe, and is not commonly associated with bunions.

**Option B:** **Hallux rigidus** is a condition where the big toe joint becomes stiff and arthritic, but it is not specifically characterized by a bony lump on the medial aspect of the first metatarsal head.

**Option C:** **Mallet toe** is a condition where the toe becomes bent downward due to a deformity of the distal interphalangeal joint, and is not related to bunions.

**Clinical Pearl / High-Yield Fact**
Bunions can be prevented or alleviated by wearing properly fitting shoes with wide toe boxes and by avoiding activities that put pressure on the toes.

**Correct Answer: C. Fifth metatarsal**