To expand on this, contraction of flexor digitorum profundus muscle results in flexion at the DIPs and PIPs. The tendon attaches at the tip of the finger, in contrast to flexor digitorum superficialis (attaches at the PIPs). Recall innervation of the forearm muscles is mostly from the median nerve, except for 1.5 muscles (ulnar half of flexor digitorum profundus and the flexor carpi ulnaris, both supplied by ulnar which makes it ez to remember yay).

But also if you got this wrong like me go read that link because it's a really nice review.

https://geekymedics.com/nerve-supply-to-the-upper-limb/

a nice review of myotome and dermatome / reflexes

Finger flexion done by:

• FDP = flexor digitorum profundus (flexes DIP)
• FDS = flexor digitorum superficialis (flexes PIP)

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Innervation:

• FDS 2/3/4/5 by median (C5-T1)
• FDP 2/3 by median (C5-T1)
• FDP 4/5 by ulnar (C8-T1)

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Our patient can't flex DIP of ring finger → FDP4 → ulnar → C8-T1.

Only possible answer we can pick is C8.

Flexor Digitorum Profundus Action: Flexes distal phalanges at distal interphalangeal joints of medial four digits; assists with flexion of hand Innervation: Medial part: ulnar nerve (C8 and T1); Lateral part: anterior interosseous branch of median nerve (C8 and T1) https://rad.washington.edu/muscle-atlas/flexor-digitorum-profundus/