**Question:** Most common trisomy among following is -

A. Trisomy 21 (Down syndrome)
B. Trisomy 18 (Edwards syndrome)
C. Trisomy 13 (Patau syndrome)
D. Trisomy 9 (not a common trisomy)

**Core Concept:**
Trisomies are chromosomal disorders caused by the presence of an extra copy of a chromosome in an individual's cells. In humans, trisomies most commonly occur due to errors during cell division, leading to three copies of chromosome number instead of the usual two.

**Why the Correct Answer is Right:**
Trisomy 21 (Down syndrome) is the most common trisomy among the options provided. Trisomy 21 occurs when an individual has an extra copy of chromosome 21, leading to developmental and physiological abnormalities. This is more common than trisomy 18 (Edwards syndrome) or trisomy 13 (Patau syndrome), which are less frequent. Trisomy 9 is not a common trisomies, as it does not appear in the list of trisomy types in humans.

**Why Each Wrong Option is Incorrect:**

**Option A (Trisomy 18):** Trisomy 18 occurs when an individual has an extra copy of chromosome 18. It is less common than trisomy 21 but still a recognized chromosomal abnormality.

**Option B (Trisomy 13):** Trisomy 13 occurs when an individual has an extra copy of chromosome 13. It is less common than trisomy 21 and 18 but still a recognized chromosomal abnormality.

**Option D (Trisomy 9):** Trisomy 9 is not considered a common trisomy in humans, making it the incorrect answer.

**Trisomy 9 is not listed among the common trisomies (21, 18, and 13)**, making it less likely to be the correct answer.

**Clinical Pearl:**

Trisomies are important to recognize as they can lead to significant developmental and health issues in affected individuals. Trisomy 21 is the most common, while trisomies 18 and 13 are less common. Recognizing these conditions can be crucial for accurate diagnosis and appropriate management of affected patients.