**Question:** A 1-month old baby presents with frequent vomiting and failure to thrive. There are features of moderate dehydration. Blood sodium is 122 mEq/1 and potassium is 6.1 mEq/1. The most likely diagnosis is:

A. Hyponatremia
B. Hyperkalemia
C. Hypokalemia
D. Hyponatraemia and Hypokalaemia

**Correct Answer:** D. Hyponatraemia and Hypokalaemia

**Core Concept:** Hyponatraemia and Hypokalaemia are two common electrolyte imbalances in infants and children, often resulting from various underlying causes. In this case, the patient presents with frequent vomiting, failure to thrive, and features of moderate dehydration, which are indicative of electrolyte imbalances.

**Why the Correct Answer is Right:** The correct answer is D because the patient exhibits both Hyponatraemia (sodium concentration less than 135 mEq/L) and Hypokalaemia (potassium concentration less than 3.5 mEq/L). These electrolyte imbalances are often caused by inadequate intake or excessive loss of these electrolytes, particularly in dehydrated patients. In this scenario, the patient has moderate dehydration, which is a common cause of these electrolyte imbalances in infants and children.

**Why Each Wrong Option is Incorrect:**

A. Hyponatraemia is not the most likely diagnosis in this case because the patient's potassium level (6.1 mEq/L) is within the normal range (3.5-5.0 mEq/L) for infants.

B. Hyperkalemia (elevated potassium levels) is not the correct diagnosis because the patient's potassium level is low, not high.

C. Hypokalaemia is not the most likely diagnosis in this case because the sodium concentration is normal, not low.

**Clinical Pearl:** Monitoring electrolyte levels, especially sodium and potassium, is crucial in identifying and treating electrolyte imbalances in infants and children. Inadequate intake or excessive loss of these electrolytes can lead to severe complications, including seizures, cardiac arrhythmias, and respiratory distress. Prompt management is essential for stabilizing the patient and preventing complications.