Okay, let's tackle this question. The user provided a question about pyloric stenosis and wants a detailed explanation for an MCQ. The correct answer is missing, so first I need to figure out what the options could be. Since the question is asking which is NOT seen in pyloric stenosis, I need to recall the typical features of this condition.

Pyloric stenosis is a condition where the pylorus becomes narrowed, leading to gastric outlet obstruction. Common symptoms include projectile vomiting, weight loss, dehydration, and metabolic alkalosis due to loss of gastric acid. The typical presentation is in infants, so maybe the options include signs like hypokalemia or hypochloremia. Also, since vomiting leads to loss of H+ and Cl-, the body retains HCO3- causing alkalosis. But hypokalemia is also common because of increased aldosterone secretion due to volume depletion.

Wait, the question is which is NOT seen. So the correct answer would be something that doesn't occur. For example, hyperkalemia is not seen because the body loses potassium via vomiting and diuresis. So if one of the options is hyperkalemia, that's the correct answer. Alternatively, maybe metabolic acidosis is a wrong option, but in pyloric stenosis, you get metabolic alkalosis. So if an option says metabolic acidosis, that's the answer.

Looking at the structure, the user wants the core concept first. Then explain why the correct answer is right, then each wrong option. The clinical pearl should highlight a key fact. The correct answer line comes at the end.

I need to make sure the explanation is thorough but concise, within the character limit. Also, use proper medical terms and structure each section as specified. Let me check if I have all the required sections and that each part is clearly labeled. Also, the wrong options need to be addressed with why they're incorrect. Let me make sure the clinical pearl is high-yield, maybe a mnemonic or a classic exam trap.

**Core Concept**
Pyloric stenosis is a gastric outlet obstruction caused by hypertrophy of the pyloric sphincter, leading to projectile vomiting, dehydration, and secondary metabolic alkalosis with hypokalemia. The loss of gastric acid (HCl) and potassium drives these electrolyte disturbances.

**Why the Correct Answer is Right**
Metabolic acidosis is **not** seen in pyloric stenosis. The condition classically causes **metabolic alkalosis** due to persistent vomiting of HCl, which depletes chloride and hydrogen ions, leading to bicarbonate retention. Potassium loss from vomiting and compensatory aldosterone secretion exacerbates hypokalemia. Acidosis would contradict the pathophysiology of this condition.

**Why Each Wrong Option is Incorrect**
**Option A:** *Projectile vomiting* is a hallmark of pyloric stenosis due to gastric peristalsis against an obstructed pylorus.
**Option B:** *Weight loss* occurs from chronic vomiting and inadequate caloric intake.
**Option C:** *Hypokalemia* results from potassium loss in vomitus and increased aldosterone-driven renal excretion.

**Clinical Pearl / High-Yield Fact**
Remember: **"Hypochloremic hypokalemic metabolic alkalosis"** is the classic electrolyte profile in pyloric st

✓ Correct Answer: C. Bilious vomiting