What medical condition is characterized by a child not gaining weight or growing properly?

Organic Failure to Thrive refers specifically to a situation in which a child does not gain weight or grow properly due to physiological or medical reasons. This condition can encompass various underlying health issues, such as metabolic disorders, chronic infections, or gastrointestinal problems that prevent the child from absorbing nutrients effectively.

In contrast, other options like malnutrition can lead to a similar lack of growth but generally refer to dietary deficiencies rather than a precise medical condition with identifiable physiological causes. Chronic illness may impact a child's growth, but it encompasses a broader range of health issues that may not directly relate to the failure to thrive. Mental health issues could affect a child’s appetite and, consequently, their growth, but they do not constitute a medical condition characterized specifically by the absence of growth from a physical standpoint. Thus, Organic Failure to Thrive is the most accurate answer because it directly links the failure to gain weight and grow with identifiable medical conditions.