**Question:** An 8 month female baby presented with 1 day history of lethargy. She had vomited several times. Her drug addict mother said she appeared "intoxicated". O/E: baby was obtunded but she was easily arousable and muscle tone was normal. RR = 60/min. Pupils are normal. BP was 112/62. Ix: Na+= 135, K+ = 4.2, Glucose 5.9 mmol/l, Cl- = 116, Urinalysis: pH = 5.0; negative for glucose & ketones, positive for calcium oxalate crystals. ABG: pH = 7.19, pCO2 = 20, PO2 = 100, HCO3 = 6.2?

A. Hyponatremia
B. Hyperkalemia
C. Hypernatremia
D. Hyperkalemia

**Correct Answer:**

**Core Concept:** Calcium Oxalate crystals are formed when the urine is acidic, and the pH is low. The presence of calcium oxalate crystals indicates low urine pH and high calcium levels, which is a sign of hypercalciuria.

**Why the Correct Answer is Right:**

The correct answer is **hypernatremia** (option C). The baby's electrolyte levels are as follows:

1. Sodium (Na+) level is 135 mmol/L. The normal range for sodium in a pediatric patient is 135-145 mmol/L.
2. Potassium (K+) level is 4.2 mmol/L. The normal range for potassium in a pediatric patient is 3.5-5.5 mmol/L.
3. Blood pH is 7.19, which indicates respiratory alkalosis, suggesting the baby might have been breathing extra oxygen or receiving extra oxygen therapy, which leads to alkalosis.
4. Blood PCO2 is 20 mmHg, which is low. This indicates respiratory alkalosis.
5. Blood PO2 is 100 mmHg, which is high. This indicates respiratory alkalosis.
6. Blood HCO3 is 6.2 mmol/L, which is low. This indicates respiratory alkalosis.

**Why the Incorrect Options are Wrong:**

A. Hyponatremia (hyponatremia in question) - The sodium level is within the normal range, so this is incorrect.
B. Hyperkalemia - The potassium level is within the normal range, so this is incorrect.
C. Hypernatremia (hyponatremia in question) - The sodium level is within the normal range, so this is incorrect.
D. Hypokalemia - The potassium level is within the normal range, so this is incorrect.

**Clinical Pearls:**

1. Respiratory alkalosis can be caused by various factors like respiratory hyperventilation, increased alveolar ventilation, and decreased alveolar CO2 levels (hypocapnia).
2