This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Unknown
Delta-sign is seen in- 
Select and explain. 
A. Petrositis, B. Acute mastoiditis, C. sigmoid sinus thrombosis, D. Glomus tumor
C: sigmoid sinus thrombosis
Answer: C: sigmoid sinus thrombosis 
Explanation: (Refer: PL Dhingra, Diseases of Ear, Nose & Throat, 7th edition, pg no. 92)

Contrast-enhanced CT scan can show sinus thrombosis by typical delta-sign.  It is a triangular area with rim enhancement, and central low density area is seen in posterior cranial fossa on axial cuts
Delta-sign may also be seen on contrast enhanced MRI