A mother brings her 5-year-old boy to see you as a General Physician. On examination, he has red eyes, dry, cracked lips and a rash on his hands and feet. He also has cervical lymphadenopathy. What is the most impoant investigation to rule out a serious complication of this condition?

**Core Concept**
The patient's symptoms, including red eyes, dry lips, rash on hands and feet, and cervical lymphadenopathy, are suggestive of measles, a viral infection caused by the measles virus. Measles is characterized by its triad of fever, cough, and a characteristic rash.

**Why the Correct Answer is Right**
The most serious complication of measles is measles encephalitis, a rare but life-threatening condition that occurs in approximately 1 in 1000 cases. Measles encephalitis is caused by the viral replication in the brain, leading to inflammation and damage to the brain tissue. To rule out this serious complication, a **lumbar puncture (LP)**, also known as a spinal tap, is essential to assess for the presence of measles virus in the cerebrospinal fluid (CSF). This investigation can help diagnose measles encephalitis and guide appropriate management.

**Why Each Wrong Option is Incorrect**
**Option A:** Complete Blood Count (CBC) - While a CBC can help detect signs of infection or inflammation, it is not a specific investigation for measles encephalitis.
**Option B:** Urine Analysis - Urine analysis is not relevant in the diagnosis of measles or its complications.
**Option C:** Chest X-ray - Chest X-ray may be useful in detecting complications of measles such as pneumonia, but it is not the most important investigation to rule out measles encephalitis.

**Clinical Pearl / High-Yield Fact**
Measles encephalitis is a medical emergency requiring prompt recognition and management. A high index of suspicion and prompt investigation are essential to prevent long-term neurological sequelae.

**Correct Answer:** C. Lumbar Puncture (LP)

✓ Correct Answer: C. Echocardiography