**Core Concept**
Rickets is a disorder caused by vitamin D deficiency, leading to softening and weakening of bones in children, resulting in characteristic skeletal deformities.

**Why the Correct Answer is Right**
In this case, the symptoms of frontal bossing (an abnormal prominence of the forehead) and wrist enlargement are classic signs of rickets. Vitamin D plays a crucial role in calcium absorption and bone mineralization. A deficiency in vitamin D impairs the body's ability to absorb calcium, leading to softening of bones (osteomalacia in adults, rickets in children). This results in characteristic skeletal deformities, including frontal bossing and wrist enlargement.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Rickets is primarily caused by vitamin D deficiency, not calcium deficiency. While calcium is essential for bone health, vitamin D is necessary for the proper absorption of calcium.
**Option B:** This option is incorrect because Marfan syndrome is a genetic disorder affecting the body's connective tissue, leading to long limbs, tall stature, and other characteristic features, but not typically frontal bossing or wrist enlargement.
**Option C:** This option is incorrect because Osteogenesis imperfecta is a genetic disorder of bone fragility, leading to short stature, blue sclerae, and other characteristic features, but not typically frontal bossing or wrist enlargement.

**Clinical Pearl / High-Yield Fact**
Rickets can be prevented by maintaining adequate vitamin D levels, particularly in children, through adequate sun exposure, diet, or supplements. It's essential to recognize early signs and symptoms to prevent long-term skeletal deformities.

**Correct Answer: D. Rickets.**