Okay, let's tackle this question about pyruvate dehydrogenase deficiency. The question is asking which statement is NOT true about this condition. First, I need to recall what pyruvate dehydrogenase does. It's an enzyme complex that converts pyruvate into acetyl-CoA, which is crucial for the Krebs cycle. A deficiency here would block this conversion, leading to a backup of pyruvate and lactate accumulation, causing lactic acidosis.

The correct answer is the exception. Common features of PDD include neurological symptoms like developmental delay, seizures, and ataxia. Hypotonia is also typical. They might have elevated lactate and pyruvate levels in blood and CSF. Treatment often involves thiamine (since it's a cofactor for the enzyme) and a ketogenic diet to bypass the blockage.

Looking at the options, if one option says it's inherited in an autosomal dominant manner, that's incorrect because PDD is usually X-linked or autosomal recessive. Another wrong option might claim that it's treated with a high-carb diet, but the opposite is true; carbs would increase pyruvate, so a low-carb, high-fat diet (ketogenic) is used. Also, if an option states that it's caused by a deficiency in vitamin B6, that's wrong because thiamine (B1) is the key cofactor here.

So the exception would be the option that contradicts these points. For example, if an option says it's autosomal dominant, that's the answer. The clinical pearl here is remembering the cofactors and inheritance patterns. Also, the treatment approach is vital for exams.

**Core Concept** Pyruvate dehydrogenase deficiency (PDD) is a mitochondrial metabolic disorder caused by impaired conversion of pyruvate to acetyl-CoA. It leads to lactic acidosis, neurodegeneration, and energy metabolism defects due to E1 subunit deficiency (often X-linked) or E2/E3 subunit defects (autosomal recessive).

**Why the Correct Answer is Right** PDD presents with lactic acidosis, hypotonia, developmental delay, and seizures. Thiamine (B1) supplementation is critical as it acts as a cofactor for the E1 subunit. A ketogenic diet is therapeutic to bypass pyruvate-to-acetyl-CoA blockage. The exception is **Option C**, which falsely claims autosomal dominant inheritance—PDD is X-linked or autosomal recessive.

**Why Each Wrong Option is Incorrect**
**Option A:** "Neurological symptoms are common" is correct—encephalopathy and ataxia are hallmark features.
**Option B:** "Lactic acidosis occurs due to pyruvate accumulation" is correct—blocked Krebs cycle entry leads to anaerobic glycolysis and lactate buildup.
**Option D:** "Thiamine-responsive in some cases" is correct—E1 subunit deficiencies often show partial thiamine responsiveness.

**Clinical Pearl / High-Yield Fact** Remember "LAD" for PDD: **L**actic acidosis, **A**utosomal recessive/X-linked, **D**evelopmental delay. Avoid high-carbohydrate diets in PDD patients, as they worsen lactic acidosis.

✓ Correct Answer: B. Decrease in blood lactate