**Question:** All the following signs could result from infection within the right cavernous sinus, except

A. Horner's syndrome
B. Proptosis
C. Diplopia
D. Anosmia

**Core Concept:**
The cavernous sinus is a paired group of venous sinuses located in the skull base, adjacent to the carotid canal. It contains the ophthalmic branch of the trigeminal nerve, the superior and inferior ophthalmic veins, and the nerves that supply the lacrimal and nasal glands. Infections in the cavernous sinus can lead to various signs and symptoms due to damage to the structures within.

**Why the Correct Answer is Right:**
Diplopia (double vision) is a result of the involvement of the oculomotor nerve, which is within the cavernous sinus. Consequently, diplopia would be present in all cases of infection in or around the cavernous sinus.

**Why Each Wrong Option is Incorrect:**
A. Horner's syndrome (ptosis, miosis, pheochromocytoma) is caused by damage to the sympathetic preganglionic fibers passing through the cavernous sinus. However, Horner's syndrome typically involves both cavernous sinuses, making it less likely to be seen in a unilateral infection.
B. Proptosis (eye bulging) is a result of inflammation, edema, and mass effect on the structures within the orbit, rather than the cavernous sinus itself. While proptosis can occur in orbital infections, it is not specific to cavernous sinus infections.
D. Anosmia (loss of smell) is a result of damage to the olfactory nerve, which is located outside the cavernous sinus. Anosmia would not be expected in unilateral cavernous sinus infections.

**Clinical Pearl:**
Infections or inflammation within the cavernous sinus can lead to various signs and symptoms, including diplopia, due to involvement of the oculomotor nerve. However, specific signs such as Horner's syndrome, proptosis, or anosmia are less likely to be seen in unilateral infections of the cavernous sinus. This is because the cavernous sinus typically contains bilateral sympathetic fibers, and the olfactory nerve is located outside the sinus. Diplopia is more likely due to inflammation and mass effect on the oculomotor nerve.