**Question:** A strict vegetarian is not getting sufficient vitamin D in his diet, and he develops osteomalacia. This disease is characterized by which of the following changes in his bones?

A. Loss of bone density
B. Increased bone pain
C. Enlarged bone size
D. Softening and deformity of bones

**Core Concept:**
Osteomalacia is a condition caused by impaired vitamin D and calcium absorption, leading to weak and soft bones. Vitamin D plays a crucial role in calcium homeostasis by promoting calcium absorption in the gut and promoting calcium deposition in bones. Osteomalacia results from inadequate vitamin D and calcium intake or impaired absorption, leading to decreased bone mineralization.

**Why the Correct Answer is Right:**
In osteomalacia, the correct answer D (softening and deformity of bones) represents the pathological changes in bone tissue due to impaired mineralization. Impaired vitamin D and calcium absorption results in decreased bone mineralization, causing bones to become soft and prone to deformity.

**Why Each Wrong Option is Incorrect:**
A. Loss of bone density: Osteomalacia primarily affects bone softening and deformity, not loss of bone density.
B. Increased bone pain: While pain may be present, it is not the primary feature of osteomalacia.
C. Enlarged bone size: Osteomalacia primarily affects bone mineralization, not bone size.

**Clinical Pearl:**
A strict vegetarian diet can lead to vitamin D deficiency, which in turn causes osteomalacia. Ensuring adequate vitamin D intake, especially from dietary sources like eggs, fatty fish, and fortified foods, is crucial for maintaining bone health. Sunlight exposure is also an essential source of vitamin D production in the skin. Regularly assessing vitamin D levels in high-risk patients, such as vegetarians, can help prevent and manage osteomalacia.

**Correct Answer:** D (Softening and deformity of bones)