**Question:** Premature baby of 34 weeks was delivered. Baby developed bullous lesions on the skin and X-ray shows periostitis. What should be the next investigation?

**Core Concept:**
The case description pertains to a premature baby born at 34 weeks, presenting with skin lesions and periostitis on X-ray. Bullous lesions in premature babies are often associated with systemic conditions, and periostitis on X-ray suggests a vitamin D deficiency.

**Why the Correct Answer is Right:**
In this scenario, considering the clinical presentation and radiological findings, the correct answer is to investigate for hypocalcemia (low blood calcium levels) and hypophosphatemia (low blood phosphate levels), as these conditions are associated with periostitis and bullous dermatitis in premature infants. Both hypocalcemia and hypophosphatemia are consequences of inadequate vitamin D levels, as vitamin D is essential for calcium and phosphate absorption from the gut.

**Why Each Wrong Option is Incorrect:**

A. Hypothyroidism: Though hypothyroidism can lead to developmental delays, it is not directly linked to periostitis and skin lesions seen in this case.

B. Disseminated intravascular coagulation (DIC): DIC is a systemic coagulation disorder and does not explain the specific skeletal and cutaneous findings in the presented scenario.

C. Infectious diseases: Infectious diseases can cause skin lesions, but periostitis on X-ray is not a typical finding in infectious diseases.

D. Congenital infections: Similar to option C, congenital infections do not typically lead to periostitis on X-ray.

**Clinical Pearl:**
Investigating for hypocalcemia and hypophosphatemia in the context of vitamin D deficiency is crucial, as early diagnosis and treatment can prevent complications, such as seizures, tetany, and rickets, which are common in premature infants with vitamin D deficiency.

**Correct Answer:**
The correct answer is to investigate for hypocalcemia and hypophosphatemia secondary to vitamin D deficiency. It is essential to initiate targeted therapy with vitamin D supplementation and calcium and phosphate replacement to prevent complications in this vulnerable population.