Arm ache while descending on drops

From a completely ignorant perspective I'd say your saddle is too high and too far forwards.

Look at your position again.

Lessen the death grip on the bars and relax. Wear mitts and get some grippier bar tape if worried about keeping control.

Do press ups and triceps dips.

Everything peted76 has said but I have a question ...

When you are descending in the drops are you gripping your bars too tightly?

The reason I ask this is because if you grip the bars too tightly you can cause your arms and shoulders to 'lock'. Depending on how your front end is set up this can cause joint, tendon & muscle pain and cramps.

So,
1) Check your bike fit.
2) Work on your core.
3) Practice your descending.
4) Get used to and comfortable to being in the drops. A lot of problems are caused by the very fact that we seldom get down into them. We spend so much time riding on the hoods that we forget about them.

Best of luck.

Caused by too much weight on your arms. You can change your set up to fix.

Simple 'core' excersises is the key to this I believe. Strengthen your core so not all the weight is on your arms (e.g. some is taken by your 'core')
Test whether you can sit in your 'drops position' and then take both hands off the bars... does your core support you?

I get it also on long decents, but that's probably more becuae I'm not used to them.