Identify the structure indicated by the arrow.

Question

BlackTom AI · Accepted Answer

The coronoid process of the ulna is an anterior, triangular projection of the proximal ulna that fits into the coronoid fossa of the humerus when the elbow is flexed.

1) Relevant knowledge points and concepts
- Proximal ulna anatomy: two key processes — the olecranon (posterior, forms the bony tip of the elbow) and the coronoid process (anterior, forms the lower border of the trochlear notch).
- Proximal radius anatomy: the radial head is a rounded, disk-like structure lateral to the proximal ulna that articulates with the capitulum of the humerus.
- Radiographic orientation: on a lateral elbow x-ray the posterior olecranon appears as the superior posterior prominence, while the coronoid projects anteriorly beneath the humeral trochlea.

2) Step-by-step reasoning
- Locate the arrow on the lateral elbow x-ray: it points to an anterior bony projection immediately below the distal humerus and forming part of the semilunar (trochlear) notch.
- Determine whether this projection is anterior or posterior: the projection is anterior. The olecranon would be posterior and superior (not where the arrow points).
- Compare to radial head: the radial head appears as a round disk laterally and slightly inferior; the arrowed structure is medial/anterior and part of the ulna, not the round radial head.
- Compare to humeral epicondyles: epicondyles are part of the distal humerus, not an anterior projection of the ulna.

3) Calculations
- None required.

4) Why the correct answer is correct
- The arrow marks the anterior triangular projection of the proximal ulna that forms the lower part of the trochlear notch — the coronoid process.

5) Why other options are incorrect
- Head of the radius: would appear as a rounded disk lateral to the ulna, not this anterior triangular projection.
- Medial epicondyle of the humerus: is on the humerus, not an ulna process under the trochlea.
- Olecranon process: is posterior/superior (the elbow’s tip), opposite side of the trochlear notch.
- Head of the ulna: is distal on the wrist, not at the proximal elbow.

6) Conclusion
The structure indicated is the coronoid process of the ulna, matching the provided answer.

Identify the structure indicated by the arrow. Single choice

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