Okay, let's tackle this question about Down syndrome. The user wants a detailed explanation for a multiple-choice question where the correct answer is the exception among the given options. First, I need to recall the key facts about Down syndrome.

Down syndrome is caused by trisomy 21, right? So the core concept here is the chromosomal abnormality. The question is asking which statement is false, so I need to think about common misconceptions or less-known facts.

The correct answer is probably a statement that's not true about Down syndrome. Let's break down possible options. Common features include intellectual disability, characteristic facial features, heart defects, and increased risk of certain conditions like Alzheimer's or leukemia. Also, there's a higher incidence in older mothers.

If one of the options says something like "caused by trisomy 18," that's wrong because trisomy 18 is Edwards syndrome. Another incorrect option might be about a different inheritance pattern, but Down syndrome is usually de novo. Maybe an option stating that it's a single-gene disorder is incorrect since it's a chromosomal disorder. Also, some people might confuse it with Turner syndrome (monosomy X).

The clinical pearl here is that Down syndrome is the most common autosomal trisomy and is associated with specific physical and medical features. The exception in the question would be the statement that doesn't align with these facts. So, if an option claims it's a sex chromosome disorder or has a different trisomy, that's the wrong one.

Now, structuring the explanation: start with the core concept about trisomy 21. Then explain why the correct answer is right, and each wrong option is incorrect. Finally, the clinical pearl about the most common trisomy and associated features. Need to make sure each part is concise and fits within the character limit.

**Core Concept**
Down syndrome (trisomy 21) is a chromosomal disorder caused by the presence of an extra copy of chromosome 21. It is associated with intellectual disability, distinct facial features, and increased risks of congenital heart defects, leukemia, and early-onset Alzheimer’s disease.

**Why the Correct Answer is Right**
The correct exception is typically a statement misattributing Down syndrome to trisomy 18 (Edwards syndrome) or 13 (Patau syndrome), or incorrectly labeling it as a single-gene disorder. Down syndrome is a **chromosomal aneuploidy**, not a monogenic mutation. Maternal age >35 increases risk due to meiotic nondisjunction, not paternal age.

**Why Each Wrong Option is Incorrect**
**Option A:** If claiming Down syndrome is caused by trisomy 18, it’s incorrect—trisomy 18 causes Edwards syndrome.
**Option C:** If stating it is an X-linked disorder, this is false—Down syndrome is autosomal.
**Option D:** If asserting it is always lethal in infancy, this is wrong—many individuals with Down syndrome live into adulthood.

**Clinical Pearl / High-Yield Fact**
Down syndrome is the **most common autosomal trisomy** at birth. Remember the “rule of three”: three copies of chromosome 21, three facial features (upslanted palpebral fissures, epicanthic folds, flattened nasal bridge),

✓ Correct Answer: D. It is the most common trisomy in humans