Noah is a 15-year-old who presents with a two-year history of deformities in the central parts of both thumbnails. 

Test your clinical know-how by taking the quiz below. If you select the wrong answer, have another go — you can try as many times as you like!

<p>Noah is a 15-year-old who presents with a two-year history of deformities in the central parts of both thumbnails.</p><p>He had previously been diagnosed with nail dystrophy but with no underlying cause identified and no specific treatment provided.</p><p>Noah is otherwise well, with no past or family history of note.</p><p>On examination, the cuticles of both thumbnails are absent, the proximal nail folds are keratinised (pictured, marked a) and the lunulae enlarged. </p><p>There is a longitudinal groove from the proximal nail fold to the distal nail. </p><p>The radial edges of the groove are sharp (marked b), and the ulnar edges appear smooth (marked c).</p><p>No other digits are affected, and there are no other examination findings of note.</p><p>During the consultation, Noah is observed to repeatedly rub the nail plates of both thumbs with the tips of his index fingers.</p>

Dr Kate Kelso is a GP and medical editor at Australian Doctor.