**Core Concept**
Poisonous snakes, also known as venomous snakes, possess specific characteristics that distinguish them from non-poisonous species. These traits include anatomical features that facilitate the delivery and efficacy of their venom.

**Why the Correct Answer is Right**
The belly scales of poisonous snakes are typically not small. In fact, they are often large and prominent, which helps in identification. This characteristic is in contrast to non-venomous snakes, where belly scales are usually small and inconspicuous. The large belly scales of poisonous snakes may aid in the spreading of their bodies for display purposes during the mating season or as a warning signal to potential predators. This is an important observation for forensic experts and herpetologists.

**Why Each Wrong Option is Incorrect**
**Option A:** Fangs present is a correct characteristic of poisonous snakes, as they possess specialized teeth designed for delivering venom into their victims. Therefore, this option is incorrect because it describes a true feature of poisonous snakes, not an exception.

**Option C:** Small head scales is also a characteristic of poisonous snakes. In many species, the head scales are smaller than those on the body, which can be an important identification feature. This option is incorrect because it accurately describes a trait of poisonous snakes.

**Option D:** Grooved teeth are a feature of poisonous snakes, particularly those in the cobra and viper families. The grooves on their teeth facilitate the spread of venom as the snake bites its prey. This option is incorrect because it accurately describes a trait of poisonous snakes.

**Clinical Pearl / High-Yield Fact**
When identifying a snake, remember the "Three H's" of snake identification: Head shape, Head scales, and Hood (or neck band). The presence or absence of these characteristics can help distinguish between venomous and non-venomous species.

**✓ Correct Answer: B. Belly scales are small**