Which of the following conditions involves rigidity of the bones in a joint?

Ankylosis refers specifically to the abnormal stiffening and immobility of a joint due to the fusion of the bones. This condition typically results from chronic inflammation, injury, or degenerative diseases, leading to the bones within a joint becoming rigid and fixed in a single position. The term itself is derived from Greek, where "ankylos" means bent or crooked, emphasizing the loss of flexibility.

The other terms do not relate specifically to joint rigidity. Ataxia refers to a lack of coordination, typically in muscle movements, but does not signify rigid bones in a joint. Aphagia denotes an inability to swallow, which is unrelated to joints or any condition affecting bone rigidity. Ascites is the accumulation of fluid in the abdominal cavity and also does not connect to joint conditions. Therefore, ankylosis is the correct and most relevant term for describing the rigidity of bones in a joint.