**Core Concept:** Gelle's test is a clinical test used to differentiate between lumbar and sacral spina bifida occulta, which are two types of spinal dysraphism. Spina bifida occulta is a spinal cord malformation that is asymptomatic and is often found incidentally during imaging studies.

**Why the Correct Answer is Right:** Gelle's test is a simple and safe method to differentiate between these two conditions. It involves asking the patient to cough while lying prone on a flat surface and then in the sitting position. In lumbar spina bifida occulta, the cough-induced pain in the lumbar region is reduced or absent when the patient is sitting, as the pressure on the affected area is relieved. In sacral spina bifida occulta, the pain remains constant irrespective of sitting or standing due to the involvement of sacral dermatomes in the condition.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because it doesn't address the specific differentiation between lumbar and sacral types of spina bifida occulta.

B. This option is incorrect because the test doesn't address the change in pain sensation caused by the pressure relief in sitting position in lumbar spina bifida occulta.

C. This option is incorrect because it is not the correct test for differentiating between lumbar and sacral types of spina bifida occulta.

D. This option is incorrect because it doesn't focus on the pressure relief phenomenon in the sitting position in lumbar spina bifida occulta, which is the key aspect of Gelle's test.

**Clinical Pearl:** In cases of suspected spinal dysraphism, Gelle's test helps to differentiate between lumbar and sacral forms of spina bifida occulta, allowing appropriate clinical management and referral to a specialist for further evaluation.