**Question:** Read the following symptoms and suggest the investigation to be done:
1. Salty sweat; many parents notice a salty taste when kissing their child.
2. Poor growth and weight gain (failure to thrive).
3. Constant coughing and wheezing.
4. Thick mucus or phlegm.
5. Greasy, smelly stools that are bulky and pale colored.

A. Chest X-ray
B. Stool analysis
C. Blood tests
D. Electrolyte level measurement

**Correct Answer:** D. Electrolyte level measurement

**Core Concept:**
The presented symptoms are indicative of Cystic Fibrosis (CF), an autosomal recessive genetic disorder affecting the exocrine glands. The primary function of the exocrine glands is to secrete substances, such as sweat, digestive enzymes, and mucus. In CF, mutations in the CFTR gene lead to the production of thick, sticky mucus, which can obstruct the exocrine glands and impair their normal function.

**Why the Correct Answer is Right:**
The symptoms described in the question are consistent with the clinical manifestations of Cystic Fibrosis, making measurement of electrolyte levels an appropriate investigation. Cystic Fibrosis is characterized by:

1. **Salty sweat:** Excessive salt loss in sweat due to abnormal CFTR protein function causing improper salt reabsorption in sweat glands.
2. **Poor growth and weight gain (failure to thrive):** Malabsorption of nutrients due to obstructed digestive glands, leading to malnutrition and growth impairment.
3. **Constant coughing and wheezing:** Excessive mucus production and its thick consistency obstruct the airways, leading to bronchial hyper-responsiveness and recurrent respiratory infections.
4. **Thick mucus or phlegm:** The thick mucus impairs mucociliary clearance, leading to chronic respiratory infections and lung damage.
5. **Greasy, smelly stools (diarrhoea and steatorrhoea):** Impaired fat absorption due to obstructed pancreatic enzymes and bile secretion from the pancreas and liver, respectively.

**Why Each Wrong Option is Incorrect:**

A. Chest X-ray: Though an imaging modality, chest X-ray is not a direct diagnostic test for Cystic Fibrosis.
B. Stool analysis: Although important in evaluating steatorrhoea, it does not confirm Cystic Fibrosis diagnosis.
C. Blood tests: Although blood tests can provide insights into the patient's overall health status, they do not specifically diagnose Cystic Fibrosis.
D. Electrolyte level measurement: Measurement of electrolyte levels in sweat is a diagnostic test for Cystic Fibrosis. Electrolyte imbalances, specifically increased sodium, chloride, and bicarbonate levels, are characteristic of Cystic Fibrosis.

**Clinical Pearl:**
Measurement of sweat chloride levels is the gold standard test for Cystic Fibrosis diagnosis. Other diagnostic tests include sweat chloride testing, which confirms the presence of elevated chloride levels in sweat (