**Core Concept**
Generalized thickening of cortical and cancellous bones reflects abnormal bone remodeling, often due to excessive bone formation. This is most commonly seen in conditions with dysregulated osteoblastic activity, particularly in infancy or early childhood.

**Why the Correct Answer is Right**
Infantile hyperostosis (also known as infantile hyperostosis of the skull) is characterized by diffuse, symmetric thickening of both cortical and cancellous bone, especially in the skull and long bones. It results from a hyperactive osteoblastic process, typically due to a congenital defect in bone metabolism, leading to rapid and uncontrolled bone deposition. Unlike Paget’s disease, which shows mixed areas of resorption and formation with a "corkscrew" appearance, infantile hyperostosis presents with uniform thickening and is most prominent in early infancy.

**Why Each Wrong Option is Incorrect**
Option A: Osteopetrosis is marked by defective osteoclast function, leading to dense, brittle bones and increased bone density, not generalized thickening due to excessive formation.
Option B: Paget’s disease shows chaotic bone remodeling with thickened and deformed bones, but the pattern is asymmetric and involves mixed resorption and formation, not uniform cortical and cancellous thickening.
Option C: Osteogenesis imperfecta is a connective tissue disorder with brittle bones and frequent fractures, not generalized thickening of bone structure.

**Clinical Pearl / High-Yield Fact**
Infantile hyperostosis is a rare, early-onset condition that mimics osteopetrosis but is distinguished by **diffuse, symmetric thickening** of bones and is often associated with **congenital metabolic disorders**. It is crucial to differentiate it from Paget’s disease due to distinct age of onset and radiographic patterns.

✓ Correct Answer: D. Infantile hyperostosis