Which visual field defect is produced by a lesion of the inferior nasal bundle of nerve fibers crossing at the optic chiasm?

Explanation

## Chiasmal Fiber Organization and Visual Field Defects **Key Point:** The inferior nasal fibers of the optic chiasm cross anteriorly and inferiorly, looping into the contralateral optic nerve before ascending to the lateral geniculate nucleus. This anatomical loop is called **Willebrand's knee**. ### Anatomy of Crossing Fibers at the Chiasm ```mermaid flowchart TD A["Optic Chiasm"]:::outcome --> B["Inferior Nasal Fibers"]:::outcome B --> C["Cross anteriorly & inferiorly"]:::outcome C --> D["Loop into contralateral optic nerve<br/>Willebrand's Knee"]:::outcome D --> E["Ascend to contralateral LGN"]:::outcome E --> F["Represent contralateral<br/>SUPERIOR TEMPORAL field"]:::outcome ``` ### Chiasmal Fiber Crossing Pattern | Fiber Bundle | Location in Chiasm | Crosses | Contralateral Field Represented | |--------------|-------------------|---------|----------------------------------| | **Inferior nasal** | Ventral/anterior | Yes | Superior temporal quadrant | | **Superior nasal** | Dorsal/posterior | Yes | Inferior temporal quadrant | | **Inferior temporal** | Ventral | No | Inferior nasal quadrant (ipsilateral) | | **Superior temporal** | Dorsal | No | Superior nasal quadrant (ipsilateral) | ### Why Inferior Nasal → Superior Temporal? 1. Inferior nasal retina sees the **superior temporal visual field** (inverted retinal image) 2. These fibers cross at the chiasm to the contralateral optic tract 3. Lesion of inferior nasal bundle → loss of superior temporal field in the **contralateral eye** **High-Yield:** Willebrand's knee lesions (e.g., pituitary tumor growing superiorly) cause **superior temporal quadrantanopia** — the classic "junctional scotoma" pattern. **Mnemonic:** **VINO** — **V**entral **I**nferior **N**asal fibers cross to the **O**pposite superior temporal quadrant.