**Core Concept**
Vitamin D deficiency rickets is a disorder caused by a lack of vitamin D in the body, leading to defective mineralization of bones in children. This results in softening of the bones, often accompanied by skeletal deformities and other systemic issues.

**Why the Correct Answer is Right**
Option B is correct as it includes two true statements about vitamin D deficiency rickets. **Option b)** X-ray knee joint is diagnostic: A knee X-ray is a useful diagnostic tool for vitamin D deficiency rickets, as it can reveal characteristic changes such as fraying of the metaphyses (the growing ends of bones) and cupping of the epiphyseal plates. **Option d)** Increased chances of respiratory tract infection: Children with vitamin D deficiency rickets are more susceptible to respiratory tract infections due to impaired immune function, which is partly related to vitamin D's role in regulating immune cell function.

**Why Each Wrong Option is Incorrect**
**Option A:** Vit. D3 given at a dose of 50–150 mg/day - This is incorrect as the recommended dose of vitamin D3 for treating rickets is typically in the range of 1,000-2,000 IU/day, not 50-150 mg/day. The units and dosage are significantly off.

**Option C:** ab - This option includes both correct statements, but it's not the correct answer because it doesn't include the additional correct statement in Option D, which is also true.

**Option D:** ac - This option includes statements that are partially correct but also contains an incorrect statement. **Option c)** Rickety rosary is tender - This is incorrect as the "rickety rosary" or the prominent knobs of the rib cage seen in rickets is typically not tender.

**Clinical Pearl / High-Yield Fact**
In vitamin D deficiency rickets, it's essential to remember that treatment involves not only vitamin D supplementation but also calcium and phosphate replacement to correct the mineral imbalance. Additionally, addressing any underlying causes of vitamin D deficiency, such as inadequate sun exposure or dietary insufficiency, is crucial for long-term management.

**✓ Correct Answer: B. bd**