Okay, let's tackle this question. The infant has lethargy, frontal bossing, and bowed legs, with X-ray showing widened epiphysis. Hmm, the key here is to think about bone development disorders.

First, frontal bossing is a sign of something affecting bone growth. Bowed legs make me think of rickets or maybe some other metabolic bone disease. The X-ray finding of widened epiphysis points towards a disturbance in the growth plate.

Rickets is a common differential for these symptoms. It's due to vitamin D deficiency, leading to impaired mineralization. That would explain the bowed legs and the epiphyseal widening. But wait, frontal bossing is more characteristic of craniotabes or other conditions? Wait, no, frontal bossing is also seen in rickets. Oh right, in rickets, the skull can have a bossing appearance because of the soft bones.

Another possibility is hypophosphatemic rickets, which is a different form but can present similarly. But the question doesn't mention other features like hypophosphatemia.

Scurvy is caused by vitamin C deficiency and can lead to bone issues, but the classic signs are different—like perifemoral pain and corkscrew hair.

Osteogenesis imperfecta is another possibility, but that's genetic and usually presents with fractures.

So the most likely diagnosis here is rickets. The X-ray finding of widened epiphysis with cupping and fraying is classic. The other options don't fit as well. Let me check the options again. The correct answer should be Rickets. The options might have other choices like Osteogenesis imperfecta, Scurvy, or Hypothyroidism, but based on the symptoms given, rickets is the best fit.

**Core Concept**
This question assesses the ability to diagnose rickets based on clinical and radiological findings. Rickets results from impaired bone mineralization due to vitamin D, calcium, or phosphate deficiency, leading to growth plate abnormalities.

**Why the Correct Answer is Right**
Rickets causes defective mineralization of the growth plates, resulting in **widened epiphysis** on X-ray. Clinical features like **bowed legs** (from weight-bearing on soft bones) and **frontal bossing** (due to craniotabes or soft skull bones) are classic. Lethargy may stem from hypocalcemia-induced tetany or systemic metabolic derangements.

**Why Each Wrong Option is Incorrect**
**Option A:** *Scurvy* (vitamin C deficiency) causes perifemoral pain, corkscrew hair, and subperiosteal hemorrhage, not epiphyseal widening.
**Option B:** *Osteogenesis imperfecta* is a genetic disorder with brittle bones and fractures, not growth plate abnormalities.
**Option C:** *Hypothyroidism* causes delayed bone age and short stature but not epiphyseal widening or bowing.

**Clinical Pearl / High-Yield Fact**
Remember the **"3 Ds" of rickets**: **D**eformed bones (bowed legs), **D**ermal calcinosis (absent), and **D**elayed dentition. X-ray shows **cupping and fraying** of metaphyses, not just widening. Differentiate from scur

✓ Correct Answer: B. Rickets