**Core Concept**
The carrying angle, also known as the cubitus angle, is the angle between the long axis of the humerus and the forearm when the elbow is fully extended. A normal carrying angle ranges from 5 to 15 degrees. Any deviation from this normal range can be considered a deformity.

**Why the Correct Answer is Right**
Cubitus varus, also known as gunstock deformity, is a deformity characterized by a decreased carrying angle. This occurs when the distal fragment of the humerus is displaced medially during the healing process after a supracondylar fracture of the humerus. As a result, the forearm is angled inwardly, decreasing the carrying angle. The decreased carrying angle can lead to difficulties in activities such as carrying a tray or a bag.

**Why Each Wrong Option is Incorrect**
**Option B:** Mannus varus refers to a deformity of the ankle, not the elbow. It is characterized by a decrease in the medial malleolus, resulting in a reduced angle between the tibia and the foot.
**Option C:** Cubitus valgus, also known as a "gunstock deformity" or "carrying angle deformity," is actually characterized by an increased carrying angle, not a decreased one. This occurs when the distal fragment of the humerus is displaced laterally during the healing process after a supracondylar fracture of the humerus.
**Option D:** Mannus valgus refers to a deformity of the ankle, characterized by an increased medial malleolus, resulting in an increased angle between the tibia and the foot.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that a decreased carrying angle can lead to difficulties in daily activities, such as carrying objects or using tools. This is because the inwardly angled forearm can cause the object to fall or become unstable.

**✓ Correct Answer: A. Cubitus varus**