**Question:** Tributary of the cavernous sinus includes all of the following, EXCEPT:

A. Iris
B. Nasal cavity
C. Orbital vein
D. Superior ophthalmic vein

**Core Concept:**
The cavernous sinus is a large, paired venous sinus located in the skull base, draining blood from the head and neck. It receives tributaries from various structures, including the eye, nasal cavity, and orbit. The superior ophthalmic vein is a key tributary of the cavernous sinus, draining blood from the eye and upper orbit. The iris, nasal cavity, and orbital vein are also tributaries, but the question asks for an exception, so we focus on the other three options.

**Why the Correct Answer is Right:**
The correct answer, **A. Iris**, is not a tributary of the cavernous sinus because the iris is part of the eye, which is supplied by the ophthalmic artery and drained by the ophthalmic vein, not the cavernous sinus. The iris is not directly connected to the sinus via veins.

**Why Each Wrong Option is Incorrect:**

**B. Nasal cavity:** Although the nasal cavity is drained by the superior and inferior meatus veins that empty into the pterygopalatine and sphenopalatine sinuses, respectively, these veins eventually join the pterygopalatine and sphenopalatine ganglia, not the cavernous sinus. The sinus receives tributaries from the eye, orbit, and skull base, but not the nasal cavity.

**C. Orbital vein:** The orbital vein is a tributary of the cavernous sinus, draining blood from the eye and upper orbit. It is a crucial structure within the orbit, connecting the eye and the central nervous system by passing through the cavernous sinus.

**D. Superior ophthalmic vein:** The superior ophthalmic vein is a tributary of the cavernous sinus. It drains blood from the eye and upper orbit, passing through the cavernous sinus to join the internal jugular vein. It plays a vital role in the drainage of blood from the eye and orbit to the central nervous system.

**Clinical Pearl:**
The cavernous sinus is essential in maintaining homeostasis and preserving the function of the eye and nearby structures. Understanding the anatomy and drainage pathways is crucial for diagnosing and treating orbital and cranial nerve disorders. The cavernous sinus receives blood from various structures, including the eye, orbit, and skull base. It plays a vital role in maintaining the health of the eye and its adjacent structures, as well as the cranial nerves that pass through it.