**Question:** Premature baby at 34wk was delivered & had bullous lesion on body. X ray shows periostitis. Next investigation is...

**Core Concept:** Bullae formation in premature infants is a manifestation of Systemic Inflammatory Response Syndrome (SIRS). Periostitis on X-ray indicates bone inflammation, which is a common complication in premature infants due to immature bone development.

**Why the Correct Answer is Right:** The correct answer is related to the underlying pathology in premature babies, where SIRS and periostitis are the primary issues. Systemic Inflammatory Response Syndrome is a critical response to infection or injury in premature infants. It can lead to a variety of organ dysfunction, including skin lesions like bullae (blisters). Periostitis is a complication in premature infants due to the immature bone development, which makes them more susceptible to bone inflammation.

**Why Each Wrong Option is Incorrect:**

A. **Peripheral blood smear:** While a peripheral blood smear can reveal abnormalities like leukocytosis, thrombocytopenia, and anemia, it does not directly address the underlying causes of bullae formation or periostitis.

B. **CBC with ESR & CRP:** These tests measure inflammatory markers but do not specifically address the skin lesions or periostitis.

C. **Ultrasound:** Ultrasound is a useful imaging modality for certain conditions, but it is not directly related to the manifestations of SIRS and periostitis in premature infants.

D. **Blood culture:** Blood culture is essential for identifying the causative organism in cases of suspected infection, but it does not directly address bullae formation and periostitis.

**Clinical Pearl:** Premature infants are at higher risk for SIRS due to their immature immune system and organ systems. Skin manifestations like bullae and bone inflammation (periostitis) are common complications in premature infants, indicating severe systemic inflammation. The correct investigation should focus on the underlying causes, which in this case are SIRS and bullae formation due to skin immaturity.