**Question:** Which of the following is not seen in Prader-Willi syndrome?

A. Hypotonia
B. Hypogonadism
C. Episodic vomiting
D. Open mouth posture

**Core Concept:**
Prader-Willi syndrome (PWS) is a genetic disorder caused by the absence of paternal genes on chromosome 15q11-q13. It results in characteristic features, including neurobehavioral abnormalities, hypotonia, and growth abnormalities.

**Why the Correct Answer is Right:**
The correct answer, **D. Open mouth posture**, is not a typical feature of Prader-Willi syndrome. While the other options are commonly seen in the disorder:

1. **Hypotonia (A):** Hypotonia is a generalized muscle weakness that is often present in infants with PWS due to the decreased muscle tone and reduced motor reflexes.
2. **Hypogonadism (B):** Hypogonadism refers to the deficiency of sex hormones (testosterone in males and estrogen in females) resulting in delayed puberty, infertility, and reduced secondary sexual characteristics. This is a relevant feature of Prader-Willi syndrome, which is why it is not listed as the correct answer.
3. **Episodic vomiting (C):** While vomiting can be a presenting symptom in infants with PWS, episodic vomiting is not considered a definitive feature of Prader-Willi syndrome, making it the correct answer.

**Why Each Wrong Option is Incorrect:**
A. Hypotonia is a characteristic feature of Prader-Willi syndrome, making option A incorrect.
B. Hypogonadism is a critical feature of PWS, making option B incorrect.
C. Episodic vomiting, although present in some cases, is not a definitive feature of Prader-Willi syndrome, making option C correct.

**Clinical Pearl:**
In Prader-Willi syndrome, hypotonia and hypogonadism are the two most prominent features. While vomiting can be a presenting symptom, it is not specific to this condition and may be seen in other disorders as well. Thus, the correct answer is option C, **Episodic vomiting**.

In summary, the correct answer is option C: Episodic vomiting. While vomiting may occur in infants with Prader-Willi syndrome, it is not a definitive feature of the condition and can also be seen in other disorders. This makes option C the correct answer, as hypotonia and hypogonadism are the primary features of Prader-Willi syndrome.