Pectus Carinatum: This condition is characterized by a protrusion of the sternum and ribs, which could cause one side of the chest to appear more prominent than the other. It's a common congenital condition that can become more noticeable during puberty.

Scoliosis: Although primarily affecting the spine, scoliosis can cause uneven rib positioning, leading to the appearance of a prominent rib on one side.
Musculoskeletal Asymmetry: Differences in muscle development or posture can lead to the illusion of a prominent rib.
Pectus Excavatum: While less likely given the description of a "prominent" rib (pectus excavatum typically involves a sunken appearance of the chest), it's possible if the question implies a relative prominence due to the contralateral side being more sunken.

Thoracic Tumor: Although rare, a tumor in the thoracic cavity could cause uneven chest wall protrusion. Early detection is crucial for treatment.
Infection or Osteomyelitis of the Rib: Infections affecting the ribs can lead to swelling and prominence.
Trauma: Previous trauma to the chest could result in a deformity causing one side to appear more prominent.

Poland Syndrome: A rare congenital condition characterized by underdevelopment of one side of the body, which can include the chest and ribs.
Jeune Syndrome (Asphyxiating Thoracic Dystrophy): A rare genetic disorder affecting the development of the bones, including the ribs, which can lead to a small, narrow chest.
Congenital Rib Deformities: Various rare conditions can affect the development of the ribs, leading to unusual appearances or prominences.