**Core Concept**
Osteomalacia is a disease characterized by the softening of bones due to impaired bone mineralization. This condition primarily affects adults and is often associated with inadequate vitamin D levels, calcium-phosphorus metabolism disorders, or chronic kidney disease.

**Why the Correct Answer is Right**
The correct answer, B. Mineralization of bone is defective, accurately reflects the underlying pathophysiology of osteomalacia. Mineralization of bone matrix, also known as osteoid, is a crucial process that involves the deposition of calcium and phosphate ions onto the osteoid, making it rigid and strong. In osteomalacia, this process is disrupted, leading to the accumulation of unmineralized osteoid and resulting in soft and fragile bones. This defect in mineralization is primarily due to a deficiency of vitamin D, which plays a vital role in regulating calcium-phosphorus metabolism and bone mineralization.

**Why Each Wrong Option is Incorrect**
**Option A:** Osteoid formation is defective - While osteoid formation is indeed impaired in osteomalacia, this statement does not accurately capture the primary defect in the disease. The main issue is not the formation of osteoid but rather its mineralization.

**Option C:** Both - This option is incorrect because it implies that both osteoid formation and mineralization are defective, which is not the case in osteomalacia. The disease primarily affects the mineralization process, not the formation of osteoid.

**Why Each Wrong Option is Incorrect (continued)**
**Option D:** None - This option is incorrect because it denies the involvement of mineralization defect in osteomalacia, which is the primary pathophysiological mechanism of the disease.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that osteomalacia is often associated with an increased risk of fractures, particularly vertebral compression fractures, due to the softening of bones. Additionally, patients with osteomalacia may present with symptoms such as bone pain, muscle weakness, and increased susceptibility to infections.

**Correct Answer Line**
✓ Correct Answer: B. Mineralization of bone is defect